If BluetoothPairingService was stopped and restarted, it would receive
a null intent, and it also would not signal correctly when it needed to
be stopped.
Since restarts can happen, check that we are still bonding when starting.
Also prevent crashes when we might be called with a null intent.
Test: initiate pairing remotely, adb am kill service, notice no crash
Bug: 36130262
Change-Id: I408811935fdb36591826f4481d103cf4799db27e
Use a foreground service to show the notification, separating dialog
creation from the receiver.
Test: pair request from remote device while the screen is off
Bug: 35234069
Change-Id: Ia3be3e8efdacf465e095af5a498ef5eb0fca83cd
