Adjustments to composer apis for HDR:

1. Add setDisplayBrightness as a display command, so that it may be
set atomically with composition updates for that display.
2. Adjust tests to set display brightness using display commands.

The setDisplayBrightness api on IComposer should be deprecated. It will
be removed in a follow-up patch.

Bug: 210151839
Test: builds

Change-Id: I2e4348e1d7f799d1744390afbb9bd206054eb933