Further fixup of migration to global settings

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