Grant compos permissions for signing

CompOS needs to read the artifacts on authfs that odrefresh has
created and write signature files for them.

(But it no longer needs to create any directories, so removed that.)

Fixes:
avc: denied { open } for comm="compsvc"
path="/data/misc/authfs/1/11/test-artifacts/...art" dev="fuse" ino=81
scontext=u:r:compos:s0 tcontext=u:object_r:authfs_fuse:s0 tclass=file
permissive=0
avc: denied { create } for comm="compsvc" name="compos.info"
scontext=u:r:compos:s0 tcontext=u:object_r:authfs_fuse:s0 tclass=file
permissive=0

Bug: 161471326
Test: composd_cmd async_odrefresh (with microdroid selinux enforced)
Change-Id: Ie02dedf1f18926cdbbd39e4a950c5aec80adee32
1 file changed