kiji-schema: Update ApiStability annotation on KijiPermissions Unstable->Experimental.

This is does not change the meaning or stability of KijiPermissions;
Unstable was deprecated in favor of Experimental.

Over the shoulder review by: ajprax
commit: https://github.com/kijiproject/kiji-schema/commit/c292fe634d026da2af2d4b71a8a57f0b5fb3c29e