Add support for brightness as a float

Change the framework to handle brightness as a float rather than an int.

Test: AutomaticBrightnessControllerTest
BrightnessConfigurationTest
BrightnessMappingStrategyTest
BrightnessUtilsTest
DevicePolicyManagerTest
DisplayManagerServiceTest
DisplayTest
DozeScreenBrightnessTest
PowerManagerTest
PowerManagerVrTest
SettingsProviderTest

Exempt-From-Owner-Approval: Changing param from int to float

Change-Id: I413641cd987c5ec8f82753c0388a33f85a9682de