MobileConnectionsRepository: register KeyguardUpdateMonitor callback on main thread

If we do this on the background thread, we trip an assert in
KeyguardUpdateMonitor, so do it on the main thread instead.

Bug: 342424366
Test: manual: set lockscreen, post notif, go to lockscreen, expand notif
Flag: com.android.systemui.scene_container
Change-Id: I737559baeb587bb94497ab7cdb5f1dadfc951fed