Further lock down app data

Assert that only apps and installd may open private app files.

Remove "open" permission for mediaserver/vold and remove their
neverallow exemption.

Test: verify no related audit messages in the logs.
Test: build
Fixes: 80300620
Fixes: 80418809
Bug: 80190017
Change-Id: If0c1862a273af1fedd8898f334c9b0aa6b9be728