Don't include inaccessible data dirs in library paths.

When constructing a classloader for a different app, don't include
it's private data dir in libraryPermittedPath if we don't have access
to it. (Which we often won't, although it depends on target SDK,
whether it has a shared UID, etc.)

This avoids generating SELinux denials at boot and other times,
e.g. when initialising WebView.

Bug: 161356067
Test: Boots, denials not seen, WebView works.
Change-Id: Ib246af979783e80fde3313f8aed6c3b7a886e6b2