Untrustworthy symlinks dereferenced by priv-apps could cause those apps
to access files they weren't intending to access. Trusted components
such as priv-apps should never trust untrustworthy symlinks from
untrusted apps.

Modify the rules and add a neverallow assertion to prevent regressions.

Bug: 123350324
Test: device boots and no obvious problems.
Change-Id: I8c4a5c9c8571fd29b2844b20b4fd1126db4128c0