Save brightness changes when mode switched to manual

Bug: 328601668
Test: Disable auto-brightness, set brightness to x, enable auto-brightness (it will change brightness to y), disable auto-brightness (brightness stays at y), reboot the device and see that the current brightness is y and not x.
Test: atest DisplayPowerControllerTest
Flag: EXEMPT bugfix
Change-Id: I5b8982b2757023cf6012642c1cd0c66f8c1bd2b1