Small cleanup or DataStore API javadoc.

There is also change in PreferenceManager.getSharedPreferences. The
original was not always returning null in case mPreferenceDataStore is
not null. That was contradictory with the java doc and conufing during
the runtime. Added tests for it.

Test: Separate CTS (2) being submitted along this one, see the topic.
Bug: b/34322039
Change-Id: I78a6758a674c3c8139bbe66ec17f2e29ca847828