Add setDarkAmount method to ClockPlugin interface.

setDarmAmount implementations provided by clock plugins
should use the dark amount to transition into and out
of the doze state on AOD. For example, the size of the
clock could shrink in the Y direction when leaving doze
to make room for the notification stack scroller.

Bug: 119035943
Test: KeyguardClockSwitchTest passes locally.
Change-Id: I9968dc2cc5ab8000ff525f3d1d06ffbf73359086