Add brightness-related `TARGET_RECOVERY_UI_` properties

Bug: 221048336
Test: combined with the other CL in same topic, observed recovery
brightness updated correctly according to variables set in BoardConfig.
(cherry picked from https://android-review.googlesource.com/q/commit:b684a13c1c85772bb9547f1d9481609d73fc6d54)
Merged-In: Iebad0753079414a5e11bce04008ee405c8d03580
Change-Id: Iebad0753079414a5e11bce04008ee405c8d03580