SELinux changes for the hasSystemFeature() binder cache property.

The binder_cache_system_server_prop context allows any user to read the
property but only the system_server to write it.  The only property with
this context is currently binder.cache_key.has_system_feature but users
will be added.

Bug: 140788621

Test: this was tested on an image with a binder cache implementation.  No
permission issues were found.  The implementation is not part of the current
commit.

Change-Id: I4c7c3ddf809ed947944408ffbbfc469d761a6043
6 files changed