Report audio-coupled haptics in Vibrator.isVibrating

Update VibratorController to set the isVibrating flag to true when the
vibrator enters external control. Starting from that point the audio
framework is free to send data to the vibrator motor directly, so the
service should reflect this state.

This will affect both the Vibrator.isVibrating and
OnVibratorStateChangedListener APIs.

Fix: 348589406
Flag: EXEMPT small bug fix
Test: VibratorControllerTest
Change-Id: I376c6c5773a3d5303bebe0a6e1c48121bf4e8ce8