Using SecureSettings to monitor settings on worker thread

Adjusting the SecureSettingsContentObserver to comply with the guidelines at go/register-observer-bg-migration.

Bug: 362737091
Test: atest com.android.systemui.accessibility
Flag: EXEMPT internal cleanup
Change-Id: Id6b24aa555fa4a25ec8f47a8b4d30301b52966cc
8 files changed