GitLab

This change adds a permission,
android.permission.DOWNLOAD_CACHE_NON_PURGEABLE.  When an app has this
permission, any downloads it requests through the public API to the
download cache will automatically become non-purgeable, i.e. they'll
never be automatically deleted by the download manager to free up
space.  This is intended for use only by the system updater.

Change-Id: I35cdd44f7e5d46bc70443d1a9743f61a51395ddb