Properly persisting brightness set to max for multiscreen devices

This also fixes regression with extra IO call when switching from high to low brightness screen on max brightness

Bug: b/330254981 b/345623960
Test: atest DisplayBrightnessControllerTest, manual testing
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:68cbb9234a50701314f4c447297b8101e7b4a424)
Merged-In: I4632e74170bb92f84e416a9425e788edcfef84e5
Change-Id: I4632e74170bb92f84e416a9425e788edcfef84e5