Register contentObserver on bg thread.

Perfetto trace shows thats that this registration is blocking the main
thread for 64 ms. We can eliminate this blocking call by putting it on a
bg thread.

Fixes: 310584954
Test: atest KeyguardClockSwitchControllerTest
Flag: NONE

Change-Id: I8c134e23dc430729a965bb2ba517c525a438a5f7
3 files changed