Add linter check to warn usage of registerContentObserver() synchronous API.

Warn the caller that the synchronous API will run on the main thread, and suggest usage of the async/suspend APIs declared in SettingsProxy and it's sub-classes instead.

Bug: 353191254
Test: atest RegisterContentObserverSyncViaSettingsProxyDetectorTest
Flag: NONE new linter check
Change-Id: I4c3af775d97f5f78d1a10a87fac1299123afb5e4