Dim screen brightness for low light clock.

Most devices do not support dim brightness mode (dbm). In this case, the
brightness of the screen should be reduced to minimum when showing the
low-light clock. This change adds an internal method to DreamService to
set brightness, which is then used by the LowLightClockDreamService.

Fixes: 419096438
Test: atest DreamServiceTest
Flag: com.android.systemui.lowlight_clock_set_brightness
Change-Id: I0cf3cf904e6da1da5cdbe6a61618aa11c10f1153