Allow mediaserver domain have getatrr perm on vendor_app_file

When running some apps in vendor partition, it report denials like:

avc: denied { getattr } for comm="Binder:901_2" path="/vendor/operator/app/Wechat/Wechat.apk" dev="sde14" ino=1707 scontext=u:r:mediaserver:s0 tcontext=u:object_r:vendor_app_file:s0 tclass=file permissive=0