Update System settings content observer to cache values.

Existing ContentObserver will cache values and overwrite
them when the value itself is modified or caller force
updates.

Bug: 149571513
Test: Wrote unit tests, mostly for caching logic.
Not meant to test ContentObserver contract for registering an
observer.

Change-Id: I12835f6c2be27ce17f65a55c51c4ef85c63b4487