Move binder calls in NotificationSettingsController to a bg thread

While looking at the traces of a Jank regression we've noticed that
registering the contentObsver for Settings is a binder call, which
takes ~5ms sometimes. Since this is happening in a hot path, part of
the ShadeListBuilder.buildList, we should move it to a bg thread.

Fixes: 314287084
Test: atest SystemUITests:com.android.systemui.statusbar.notification.row.NotificationSettingsControllerTest
Test: observe a perfetto trace of posting, and cancelling Notifications
Flag: NONE

Change-Id: Ib2581e15927ce28352e334af3989da499d25e28d