Move global settings observer registration to bg thread

We shouldn't be running these Binder calls on main thread, especially in
constructors. Use async version which runs on a bg thread.

The calls aren't race-condition sensitive so Async version is ok.

Bug: 327558308
Flag: EXEMPT bugfix
Test: presubmit
Change-Id: Id9d94fbc140f8a8d24e3331589d588da56c59430