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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:23b54317ee0ff9339a86f42413f04a0cdc639601)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:2d9307170785437e72b4422e12db19db5c01b749)
Merged-In: I0cf3cf904e6da1da5cdbe6a61618aa11c10f1153
Change-Id: I0cf3cf904e6da1da5cdbe6a61618aa11c10f1153