Export stub sources as part of the java_sdk_library

Minor refactoring of checkMergeZip(string) -> checkMergeZips(...string)
to allow testing of multiple merge zips.

Bug: 153443117
Test: m nothing
Merged-In: I8db00f611ced15f8476ba16f2834a72e8c913596
Change-Id: I8db00f611ced15f8476ba16f2834a72e8c913596
(cherry picked from commit 3d1248ce)