Fix race condition in Ringer

Race condition is as follows:
Initially: mIsVibrating == false

Thread 1: mVibrator.vibrate (maybeStartVibration)
Thread 2: checks mIsVibrating == false, doesn't bother stopping it
Thread 1: sets mVibrating = true
Thread 2: *never stops the vibrator*

Fix is to set mIsVibrating to true before actually starting the ringer.

Fixes: 140013629
Test: manual
Change-Id: I1eb0ee02ae54494691499e1c4fbf4dd6ae7760b5