Add @SettingsBackground dispatcher

The purpose of this executor is to allow code to dispatch operations to
background threads which are order sensitive (e.g. register/unregister
calls). Currently this is backed by a single background Looper which
runs sequentially.

Bug: 350930202
Bug: 327558308
Flag: EXEMPT no code changes in existing code
Test: n/a
Change-Id: Ie9257dc3afe6694c53418f9d4594edce4a650744