Add ota_package_file label for OTA packages.

Allow priv_app, uncrypt, update_engine to access the OTA packages at
/data/ota_package (both A/B and non-A/B). GMSCore (priv_app) checks
the existence of the folder, and downloads the package there if present.

Bug: 28944800
Change-Id: I3c0717861fce7f93b33874a99f6a4a55567612a5