In particular, make it quite clear that the set returned by
getStringSet() must not be modfied by the app, period; and
add a similar caution about the map returned via getAll().
Also, fix a bug that could lead to unexpected data being committed
if the set instance passed to putStringSet() was mutated by the
app after that call (including mutations after commit() was
invoked).
Bug 7312641
Change-Id: If9a1be1b0669ac879ff7a7dc67a8805548ea10cc
01ed79c578