Revert "Remove enableSystemApp methods."

We need to go back to uninstalling system apps so we can
re-install non-default system applications through Play.

This reverts commit e3dbcd138c07f2d32ac84229d0a49052cc18d424.

Change-Id: I0a7af094614c4a10800971c82e10571f7312e079
4 files changed