Propagate stubs to the SDK for libraries that have them.

Necessary to make the APEX build logic treat the libraries as API
boundaries rather than dependencies to bundle.

The .so files in the snapshots are the compiled stub libraries in this
case. They are strictly speaking redundant since they can be generated
from the .map.txt files in the snapshots, but doing that would require
extending the cc_prebuilt_library(_shared) module types with a full
compiler pass etc, and that would break a lot of assumptions in the cc
package.

Test: m nothing
Test: Create an SDK snapshot with Bionic libs, drop it into a
  master-art tree without bionic/ in it, build ART APEXes, and check
  that the Soong phase completes (specifically no errors about various
  APEX libs requiring libc that is not available to them).
Bug: 152481980
Change-Id: I31b928e6261198b6dd6f6b17196e714f07b64172