Vibrator: Allow priority vibrations in low-power mode for accessibility.

This is mainly for Wear devices that don't have speakers.

Bug: 37543478

Test: Manual:
1. Activate low power mode `adb shell dumpsys battery unplug && adb shell settings put global low_power 1`
2. Then see if Alarm/Timer will vibrate.
3. Send a Hangout Test notification, make sure it does NOT vibrate.

On phones there should be no change. Alarms will not vibrate.

Change-Id: Ia6dffa7e528886e7c84390dbb33b44f298c7afa4