Store Brightness Per Display

This changes stores a brightness value per display.

Add brightness value item to persistent data store.
Add BrightnessSettingController to save and retrieve brightnesses.
It is indexed by primary display device of the concerned logical display.
Add method in DisplayManager to expose this.

Bug: 176985835
Bug: 147415200
Bug: 175286226

Test: manual, check "adb shell dumpsys display | grep -A10 Persistent" shows brightness values for each display attached.
Change-Id: Ib8557f8a0f45a5bfb0810967f678015d3f121bb9