Add the thread-safe protection for PowerAllowlistBackend if possible

Add some thread-safe protection if possible, since the
PowerAllowlistBackend is designed as single instance, but multiple
modules in the Settings will use it, both in the background and main
thread to cause the potential race conditions.

Fix: 340029244
Test: presubmit
Change-Id: I3b9e76889db7807496e38371d735ca41160be88e