Stop exporting java_sdk_library libs in the snapshot

The libs property in java_sdk_library is not safe to export as it can
contain libraries that contain implementation specific classes. No
snapshot appears to need this capability but if it does become
necessary then either it will need to use the existing stub_only_libs
property or a new one will need to be added.

Bug: 157884619
Test: m nothing
Change-Id: I60a069177dbee4070d311b23d25f4eb3c5e7ea13