Make flags related for "external services" public

ServiceInfo.FLAG_EXTERNAL_SERVICE is set when the service is
exported with the new public manifest attribute "android:externalService"

Context.BIND_EXTERNAL_SERVICE is used by clients when they
are binding such service.

Bug: 22084679
Bug: 21643067
Change-Id: Ifd708f90b47a1571cd54c5ec9fc217f0567d5ff4