Revert "Add async register/unregister APIs in Settings/UserSettingsProxy."

This reverts commit 4ffcd158428429b021509c4b87265f58f2c96e4d.

Reason for revert: b/345190406

Bug: 345190406
Bug: 345190802
Test: make SystemUIGoogle
Flag: NONE revert commit
Change-Id: I0c27c4062da89ea617e18296a4ae1f59db68d582
9 files changed