Make sure only SYSTEM can call notifyStaged(Apk)Session

This is sufficient to prevent malicious callers from enabling
rollback for a session that they otherwise wouldn't be allowed
to enable rollback for.

Fixes: 128669197
Test: atest StagedRollbackTest
Change-Id: I9fa8527fadbbfd58cdee0479cae97ca11a0efd9d
1 file changed