Going forward flags will also be used instead of legacy flags for
flagging changes that do not involve new APIs. Clean up the build rules
to move them from FlaggedApi.bp to the main Android.bp (since there are
no branch differences anymore), and fix the comment.
Test: m
Flag: EXEMPT: refactor
Change-Id: I46ec52dfaf9dbef55364e2def30d97ab9bce55ed