Move all binder calls to PowerManger#userActivity to bg thread.

This gets invoked on the main thread while we transition from lockscreen -> bouncer and possibly a few other CUJs.

Bug: 404201117
Flag: com.android.systemui.bouncer_ui_revamp
Test: verified with trace that binder calls don't happen on main thread anymore.
Change-Id: If48751b5dcc315f8b1a306d9755c5d252a0a801a