java_sdk_library: Only expose impl jars when they are built

The implementation jars are not built if api_only: true. In that case
the jar paths must not be exposed implicitly as they will be nil and
they will just be ignored.

This change ensures that stubs are returned when the implementation
jars are not built.

Bug: 155164730
Test: m droid
Change-Id: Ic495982a5dcb2754916260e7d91d921a5da288ae