Update keyguard flow to always be on main thread.

KeyguardUpdateMonitor.registerCallback will throw an execption if not on
the main thread. Therefore this change enforces that the flow is always
collected on the main thread, even if the consumer tries to collect it
in a background thread.

Bug: 319331389
Test: atest KeyguardRepositoryImplTest
Flag: NA
Change-Id: I33482ed5797e87888dec55201ec650ccc89b7b38