Complete Initialization of ClockRegistry in singleton provider

This ensures we've set the flag right when the registry is created. It
also allows us to disable plugin loading when the flag is flipped off.
This ensures we won't cause syshealth isues for T.

Bug: 229771520
Bug: 254235245
Test: Manually verified clocks work in both flag states
Change-Id: I64c0f226db11f46922c44f2cc68be1efb26b06bb