OPP: Fix the NPE in OPP service when stop() is called before start().

Due to start() is not called, mNotifier is not initialized then
mNotifier.cancelNotifications() causes NPE when handling the
STOP_LISTENER message. In the stop(), add a check of the
sBluetoothOppService has been initialized before sending the STOP_LISTENER
message.

Bug: 132860582
Test: make, force not handling start() and perform Bluetooth/Airplane mode
on/off tests.

Change-Id: Ieef1a65455b03a065b1a3c3dac0a5c9e3dd0bb09