Multiuser support in ClockRegistry

This change correctly supports settings for the currently active
user, and properly switches when a user switch occurs. It also takes
the Settings.Secure binder calls off the main thread.

Bug: 267535577
Bug: 265061202
Bug: 267372164
Test: Checked multiuser scenario on device
Change-Id: I467212aa452ec3f7fcfb74f4bf60be5805803672