Replace default clock with FlexClockView

Hook DefaultClockProvider up to FlexClockView.

Turning the flag on and having the default clock selected now utilizes
the FlexClockView

Fixes: 372479433
Test: manual - enable flag and observe that the default clock uses FlexClockView
Flag: com.android.systemui.clock_reactive_variants
Change-Id: I170519260d998476ef9380b18a8ef11e0ba0e1eb