SCREEN_BRIGHTNESS - map 255 to current max

Since the current max can change for brightness due to
high-brightnes-mode (HBM), make it so that 255 is always mapped
to the current max, which can be different when in HBM or normal
brightness mode.

Bug: 187784093
Test: atest HighBrightnessModeControllerTest
Test: manually verify 255 is used for max when in HBM or not.
Change-Id: Ib0631fc12de0e8e4c7c638901d82c362e79944c3