Make ClockRegistry and its provider singleton

`ClockRegistry`, 'ClockRegistryProvider` and `PluginManager` should all
be singleton. Considering ag/23407197, keep a set of `LifecycleOwner`
whenever the registry associated with a new owner, and only unregister
listeners when the last owner leaves.

Bug: 284535120
Test: set small/dynamic clock with WPP resume/destroy
Change-Id: I8c8d2899c1d223fb3171506f6f316393086e02ca