Unregister listeners and reset wake lock

The ContentProvider might be recreated by the system, so we it's
not necessarily a singleton. We need to cleanup listener registration
and alarms before reattributing the static variable.

Fixes: 135344397
Fixes: 135582651
Test: bind and unbind slice various times
Test: atest KeyguardSliceProviderTest
Change-Id: I9bc993f372a8c05258f1778eb3d415af04544714