sepolicy: allow access to vndk-stable libs

Vndk-stable libs are system libs that are used by same process HALs.
Since same process HALs can be loaded to any process, so are vndk-stable
libs.

Bug: 37138502
Test: none, because the directory is currently empty and thus this is
no-op. sailfish builds and boots.

Change-Id: I67a2c8c2e4c3517aa30b4a97dc80dc2800e47b5a