Add Settings.(System/Secure).putStringOverrideableByRestore

Bug: 144838882
Test: 1) SettingsStateTest
      2) Manually change settings on the device; verify isModified flag
         is set; reboot; make sure changes are persisted.

In ag/10056253 logic was introduced to make sure settings modifications
are not overridden by restore. However, some callers still need to use
the old behavior. Modification made through
Settings.System.putStringOverrideableByRestore will be overridden by
restore.

Change-Id: I91084ef61711a77ff17b2a5ee1996fa15cb17e73