fix(#699): apply pre-PR review, never block the main thread in the backstop

Gemini review (gemini-2.5-pro, docs/llm-consultations) flagged that the
5 s backstop acquired mMethodCallMutex blockingly on the main looper, an
ANR class. It now tryAcquires and re-posts itself after 250 ms when a
callback holds the mutex. The other two findings were verified and
rejected with reasons recorded in the PR body: the disconnectAllDevices
race is pre-existing upstream (and naive mutex acquisition there would
deadlock the flutterRestart path, which already holds the semaphore),
and the untranslated new strings follow this repo's standing l10n
process.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/flutter_blue_plus_android/android/src/main/java/com/lib/flutter_blue_plus/FlutterBluePlusPlugin.java

@@ -816,33 +816,43 @@ public class FlutterBluePlusPlugin implements
                     if (mConnectedDevices.get(remoteId) == gatt) {
                         final BluetoothGatt disconnectingGatt = gatt;
                         final String backstopRemoteId = remoteId;
-                        new Handler(Looper.getMainLooper()).postDelayed(() -> {
-                            boolean stillTracked;
-                            try {
-                                acquireMutex(mMethodCallMutex);
-                                stillTracked = mConnectedDevices.get(backstopRemoteId) == disconnectingGatt;
+                        final Handler backstopHandler = new Handler(Looper.getMainLooper());
+                        backstopHandler.postDelayed(new Runnable() {
+                            @Override
+                            public void run() {
+                                // Never block the main thread on the mutex (ANR
+                                // risk, pre-PR review): if a callback holds it,
+                                // re-check shortly instead of waiting.
+                                if (!mMethodCallMutex.tryAcquire()) {
+                                    backstopHandler.postDelayed(this, 250);
+                                    return;
+                                }
+                                boolean stillTracked;
+                                try {
+                                    stillTracked = mConnectedDevices.get(backstopRemoteId) == disconnectingGatt;
+                                    if (stillTracked) {
+                                        log(LogLevel.WARNING, "disconnect not confirmed after 5s, force-closing: " + backstopRemoteId);
+                                        mConnectedDevices.remove(backstopRemoteId);
+                                        mCurrentlyConnectingDevices.remove(backstopRemoteId);
+                                        try { disconnectingGatt.close(); } catch (Exception e) { /* best effort */ }
+                                    }
+                                } finally {
+                                    mMethodCallMutex.release();
+                                }
                                 if (stillTracked) {
-                                    log(LogLevel.WARNING, "disconnect not confirmed after 5s, force-closing: " + backstopRemoteId);
-                                    mConnectedDevices.remove(backstopRemoteId);
-                                    mCurrentlyConnectingDevices.remove(backstopRemoteId);
-                                    try { disconnectingGatt.close(); } catch (Exception e) { /* best effort */ }
+                                    // random number defined by this fork, alongside
+                                    // flutter blue plus's own 23789258 cancel code.
+                                    int bmForceClosedErrorCode = 23789259;
+
+                                    // see: BmConnectionStateResponse
+                                    HashMap<String, Object> backstopResponse = new HashMap<>();
+                                    backstopResponse.put("remote_id", backstopRemoteId);
+                                    backstopResponse.put("connection_state", bmConnectionStateEnum(BluetoothProfile.STATE_DISCONNECTED));
+                                    backstopResponse.put("disconnect_reason_code", bmForceClosedErrorCode);
+                                    backstopResponse.put("disconnect_reason_string", "disconnect not confirmed, force-closed");
+
+                                    invokeMethodUIThread("OnConnectionStateChanged", backstopResponse);
                                 }
-                            } finally {
-                                mMethodCallMutex.release();
-                            }
-                            if (stillTracked) {
-                                // random number defined by this fork, alongside
-                                // flutter blue plus's own 23789258 cancel code.
-                                int bmForceClosedErrorCode = 23789259;
-
-                                // see: BmConnectionStateResponse
-                                HashMap<String, Object> backstopResponse = new HashMap<>();
-                                backstopResponse.put("remote_id", backstopRemoteId);
-                                backstopResponse.put("connection_state", bmConnectionStateEnum(BluetoothProfile.STATE_DISCONNECTED));
-                                backstopResponse.put("disconnect_reason_code", bmForceClosedErrorCode);
-                                backstopResponse.put("disconnect_reason_string", "disconnect not confirmed, force-closed");
-
-                                invokeMethodUIThread("OnConnectionStateChanged", backstopResponse);
                             }
                         }, 5000);
                     }