The Settings.System.STAY_ON_WHILE_PLUGGED element should have been
migrated to the global table, but wasn't. This CL does a couple of
things around dealing with this:
(1) Tidies up the migration tables outright, so that they correctly
reflect the intended final state
(2) Introduces the option of doing a key migration only if the element
has not yet been moved to the new table, to allow for safe retry-
-with-ignore. This will make it easy to make any future alterations
to the global vs per-user association of individual elements
(3) Migrates the STAY_ON_WHILE_PLUGGED element if it hasn't been already.
Bug 7126575
Change-Id: Ic5fa9ba45f11b09270bd5bc94c26fbbd84abc749
9219874be9