Selinux: Give runas permission to read system_data_file links

Run-as is running a command under an app's uid and in its data
directory. That data directory may be accessed through a symlink
from /data/user. So give runas rights to read such a symlink.

Bug: 66292688
Test: manual
Test: CTS JVMTI tests
Change-Id: I0e0a40d11bc00d3ec1eee561b6223732a0d2eeb6