Move SettingsProxyExt content observer registration call on bg thread.

Migrate content observer  registration/de-registration calls to async
APIs. This will help reduce jank caused by main thread being blocked
during corresponding binder calls.

Bug: 355389014
Test: atest SettingsProxyExtTest
Flag: com.android.systemui.settings_ext_register_content_observer_on_bg_thread
Change-Id: Ie31ad5e1d2455d0da1e1f0f1bdd7286f12850416