Bluetooth airplane listener: Check for null

Check mAirplaneHelper != null before accessing it.  Note that this can
be null if Settings.Global.RADIO_BLUETOOTH is enabled in
AIRPLANE_MODE_RADIOS.  We don't need to disable Bluetooth if it's
allowed in airplane mode.

Test: atest FrameworksServicesTests
Bug: 174254527
Change-Id: I93ae4ec6b75b7ffb8e75848d7779ab64593221bd
(cherry picked from commit b49f866054)

services/core/java/com/android/server/BluetoothAirplaneModeListener.java

@@ -127,7 +127,9 @@ class BluetoothAirplaneModeListener {
             }
             return;
         }
-        mAirplaneHelper.onAirplaneModeChanged(mBluetoothManager);
+        if (mAirplaneHelper != null) {
+            mAirplaneHelper.onAirplaneModeChanged(mBluetoothManager);
+        }
     }
 
     @VisibleForTesting