Add the mutability flag to BluetoothDevicesSlice.

Starting Android S, all PendingIntent has to have a explicit mutability
flag set.

Fixes: 172207505
Fixes: 172811893
Test: Enable BluetoothDevicesSlice and play on it.
Play on the volume panel.

Change-Id: I618455de683d619fe300309e31864b3c64f12ad1