Fix the non-working bluetooth slice in the Assistant.

The bluetooh slice has an inline control, which means the PendingIntent
of the slice action should be mutable. Update the mutability flag to fix
this issue.

Fixes: 171521962
Test: Use voice command to turn on bluetooth and the bt slice should
work properly.

Change-Id: I488eef5c94b2bf66a67fb97b4102d5da2971bf04