[Pod] Make SystemClock a @Singleton, pull into separate Dagger module

For pods, we want to ensure all production code only references
interfaces, not implementations. The last use of SystemClockImpl in
production code in AOSP was in UnfoldTransitionModule, so this CL
changes that to SystemClock.

But to do that, SystemClock has to become an @Singleton instead of an
@SysUISingleton because UnfoldTransitionModule is part of GlobalModule,
not SystemUIModule.

Bug: 307607958
Flag: EXEMPT build change
Test: mp sysuig, verify LogBuffers & other classes still have the right
time
Test: verify via logging only one SystemClockImpl is created for AOSP
classes

Change-Id: Ib1f7f5648b44ad695a963ea931f1c959e20d6715