At startup, we check with PackageManager whether a system service is
available before attempting to load it. A system service is available
if its associated feature (similar to hardware features) is present.
This does not remove unavailable services from the compiled jar.
Change-Id: I13571805083aa4e65519a74acb52efd17b9fb3d7
Refactored the directory structure so that services can be optionally
excluded. This is step 1. Will be followed by another change that makes
it possible to remove services from the build.
Change-Id: Ideacedfd34b5e213217ad3ff4ebb21c4a8e73f85
