Persist the nit brightness for the default display

Store the brightness for the default display in nits in persistent data store so that it can be persisted if the device reboots or the display device changes.

Bug: 258455654
Test: adb shell dumpsys display | grep PersistentDataStore
Test: adb pull /data/system/display-manager-state.xml
Test: atest PersistentDataStoreTest
Test: adb shell dumpsys display | grep mPersistBrightnessNitsForDefaultDisplay
Change-Id: I20a0a686ee1e4e945fbcd99d5b996534f8ef93b7
Merged-In: I20a0a686ee1e4e945fbcd99d5b996534f8ef93b7