CE: flush the staged brightness to HWC before power off

After SurfaceFlinger receives the brightness commands, the commands are
sent to HWC until the next frame presents. However, no following frames
are present after the power mode is set to off, and HWC cannot receive
the last brightness commands.

This change forces SurfaceFlinger to flush the staged brightness to HWC
prior to execution of power off.

Bug: 239908529
Test: suspend and resume repeatedly
Change-Id: Ia8fb04399e757de16e06e6516d86bdfcfabd5a2e