Fix displayed brightness value in settings.

Change BrightnessLevelPreferenceController to use float values for
brightness instead of the int system.

Change tests to use correct values and floats.

Bug: b/150099686

Test: m -j99 RunSettingsRoboTests
ROBOTEST_FILTER="com.android.settings.display.BrightnessLevelPreferenceControllerTest",  manual

Change-Id: I1a6e06b4f5532b641d766f770be351b8ca827664