Change the override brightness values to use float instead of an int in
PowerManagerService.
Bug: 150671605
Test: WmTests:RootWindowContainerTests
FrameworksServicesTests:PowerManagerServiceTest
CtsOsTestCases:android.os.cts.PowerManagerTest
FrameworksCoreTests:android.os.PowerManagerTest
Change-Id: I97e97e1500dac7f8beb112812d1347fa420afbf7