Add external services, a way to run isolated processes as a different package.

This adds android:externalService boolean attribute to <service>. If that
attribute is true, then bindService() may be called with
BIND_EXTERNAL_SERVICE to create the new service process under the calling
package's name and uid. The service will execute the code from the package in
which it is declared, but will appear to run as the calling application.

External services may only be used if android:exported="false" and
android:isolatedProcess="true".

Bug: 22084679
Bug: 21643067
Change-Id: I3c3a5f0ef58738316c5efeab9044e43e09220d01