TintController: add lock to fix NPE problem.

This problem is caused by concurrent operations of the mIsActivated object by threads, such as the following situation:
1 Binder call or Background thread. In CDS service, binder all the isReduceBrightColorsActivated method and call the isActivated method.

2 Display thread. The call chain onUserChanged -> tearDown -> setActivated(null).

Test: OEM monkey test
Flag: EXEMPT bugfix
Bug: 377834826
Change-Id: Id48e7986a98ec81769e587344b19ef7e509db0e4