Add droidstub modules for Soong java testing

Previously, only the essential java_api_library and
java_api_contribution modules were being added to the template bp file
for testing purpose. However, since the child change aosp/2640275
adds droidstubs that generates the java_api_contribution as the
dependency of the java_api_library modules, not adding the droidstubs
modules to the template bp file will lead to missing dependency errors
in Soong test cases that tests the from-text generation &
java_api_library functionality.

To prevent this, this change adds the droidstubs modules instead of the
auto generated java_api_contribution modules to the template bp file to
more closely align with the real life behavior.

Test: m nothing
Bug: 288624417
Change-Id: I6a25e2f6c5f1281e96eca15aa5eec7417635df3f