Refactor Settings/UserSettingsProxy APIs with 'Sync' suffix.

Rename register/unregister content observer APIs with 'sync' suffix.
This is to denote that these APIs are synchronous. New async APIs will
be added in later CLs with no suffix, which will be the default APIs for
future use.

Test: make SystemUIGoogle
Bug: 342974268
Flag: NONE API name refactor
Change-Id: I860b1595e54d4914835e3e8fc6923d02f1d49fde
53 files changed