Set brightness percentage

New API that can take different brightness value representations. One of them is percentage which matches what the brightness slider shows.

Shell command to get brightness percentage for CTS test.

Flush the background looper in DisplayManagerServiceTest to fix the presubmit failure where BrightnessTracker keeps running in the background.

Bug: 319865623
Test: atest DisplayManagerServiceTest, adb shell cmd display get-brightness 0 percentage
Flag: com.android.server.display.feature.flags.set_brightness_by_unit
Change-Id: Icff0870328da4060868f86f77a18bfca1a9f47f2