The current neverallow rule (compile time assertion)

  neverallow { domain -gatekeeperd -system_server } gatekeeper_service:service_manager find;

asserts that no rule is present which allows processes other than
system_server from asking servicemanager for a gatekeeperd token.

However, if system_server leaks the token to other processes, it may
be possible for those processes to access gatekeeperd directly, bypassing
servicemanager.

Add a neverallow rule to assert that no process other than system_server
are allowed to make binder calls to gatekeeperd. Even if another process
was to manage to get a binder token to gatekeeperd, it would be useless.

Remove binder_service() from gatekeeperd. The original use of the
binder_service() macro was to widely publish a binder service.
If this macro is present and the calling process has a gatekeeperd
binder token, it's implicitly possible for the following processes
to make a binder call to gatekeeperd:

 * all app processes
 * dumpstate
 * system_server
 * mediaserver
 * surfaceflinger

Removing binder_service revokes this implicit access.

Add explicit access for system_server to make binder calls to
gatekeeperd.

Add explicit access for gatekeeperd to make calls to keystore.
This was implicitly granted via binder_service() before, but now
needs to be explicit.

Change-Id: I23c1573d04ab670a42660d5922b39eecf4265b66