Save brightness into persistent data store using a handler

Bug: 221393932
Test: atest PersistentDataStoreTest
Test: Manually change brightness, run 'adb shell dumpsys display | grep -A30 PersistentDataStore' and check that the brightness value has updated. Also reboot the device and check that the brightness value stays the same.
Change-Id: I3d59017198c70fcfd99d55333b92333ad1f16ec3
Merged-In: I3d59017198c70fcfd99d55333b92333ad1f16ec3