[Android] Fix NullPointerException in AudioModeModule

android/sdk/src/main/java/org/jitsi/meet/sdk/AudioModeModule.java

      * Only used on Android < M. Runs on the main thread.
      */
     void onBluetoothDeviceChange() {
-        if (bluetoothHeadsetMonitor.isHeadsetAvailable()) {
+        if (bluetoothHeadsetMonitor != null && bluetoothHeadsetMonitor.isHeadsetAvailable()) {
             availableDevices.add(DEVICE_BLUETOOTH);
         } else {
             availableDevices.remove(DEVICE_BLUETOOTH);