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
Change-Id: I4632e74170bb92f84e416a9425e788edcfef84e5
Flag: EXEMPT bugfix
4 files changed