Build updatable-bcp-packages.txt from prebuilts in sdk snapshot

Previously, the java_import and java_sdk_library_import modules did not
make their permitted_packages available to the rule that generates the
updatable-bcp-packages.txt file. This change corrects that.

Bug: 193763688
Test: m nothing
      - Added unit tests, which all failed and then fixed the tests.
Change-Id: If0706e4551a331b48d383123088e63924dded48b