Add the mutability flag to the pendingIntent.

Starting Android S, PendingIntent should have a explicit mutability flag
to avoid vulnerability.

Fixes: 172207853
Test: 1. Make AdaptiveSleepSlice available on the homepage and click on it.
2. Play with the volume panel.

Change-Id: I9cd428b17f44f5215d8c06331d12a17f997ca557