Define SELinux policy for RELRO sharing support.

Define a domain and appropriate access rules for shared RELRO files
(used for loading the WebView native library). Any app is permitted to
read the files as they are public data, but only the shared_relro
process is permitted to create/update them.

Bug: 13005501
Change-Id: I9d5ba9e9eedb9b8c80fe6f84a3fc85a68553d52e