Bluetooth: stop pairing service correctly

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