Create sepolicy for allowing system_server rw in /metadata/staged-install

Bug: 146343545
Test: presubmit
Change-Id: I4a7a74ec4c5046d167741389a40da7f330d4c63d
Merged-In: I4a7a74ec4c5046d167741389a40da7f330d4c63d
(cherry picked from commit be5c4de29fe7ead7fa55ab6865d9f397dd179a30)
8 files changed