Allow platform_app access to keystore.

Encountered when certinstaller tries to talk to keystore:
ComponentInfo{com.android.certinstaller/com.android.certinstaller.CertInstaller}: java.lang.NullPointerException: Attempt to invoke interface method 'int android.security.IKeystoreService.test()' on a null object reference

Address the following denial:
avc:  denied  { find } for service=android.security.keystore scontext=u:r:platform_app:s0:c512,c768 tcontext=u:object_r:keystore_service:s0 tclass=service_manager

Bug: 19347232
Change-Id: I35b46da3c78b384cf04216be937c6b5bfa86452d