Avoid unnecessary SharedPrefences disk writes.

Apps commonly edit + commit redundant changes to their
SharedPreferences, not checking the existing values.  Rather than
force all apps to double-check that their settings writes aren't
redundant, we should just make .commit() faster (avoiding the disk
write) when the file already exists on disk and no effective changes
were made.

Change-Id: I7edbd0d3ace5b69b7af6d12c39797c8b7f86230b