Readback test support for per-layer dimming

Dimming is only verified if setting display brightness is supported. The
test then does:
* Sets the display brightness to max, which is the same nit level that
is found in the display xml file
* Dims a layer by 10%

Bug: 213493262
Test: builds
Change-Id: I3b552682276be26fefd7ae5586ba42af0e574e31