Loosen system app data neverallows

We don't want to prevent access from vendor platform apps to system app
data. The issue with the referencing system_app explicitly in
neverallows is that vendor platform apps which need sandboxes similar to
system_app cannot be placed under system_app without modifying the
policy for all platform apps.

Test: mmm system/sepolicy
Change-Id: Ic0052602c31be4d74b02eeea129e2d8bfbd9c8d3