VirtualDisplay brightness API council feedback changes

 - Public instead of system API
 - Use a dedicated listener instead of reusing VirtualDisplayCallback
 - Drop "requested"

Change-Id: Ifc8969e7c79694e96a84513a51777e89cb2a1d5c
Fix: 377656048
Test: presubmit
Flag: android.companion.virtualdevice.flags.device_aware_display_power
9 files changed