Simplify RealLocationTimeZoneProviderProxy

At the time of writing RealLocationTimeZoneProviderProxy, ServiceWatcher
could not accept a Handler. Now it does. Small updates to comments /
annotations made here too.

Test: build / boot / treehugger
Bug: 152746105
Change-Id: I1602dab7c31f07b0f8eb8319f84f56f7ad0f72e3