Default to not creating stubs for droiddoc

This will allow us to remove `create_stubs: false` throughout the
codebase easily, then remove the functionslity to create stubs from
droiddoc.

Test: m docs
Test: m checkapi
Test: m updateapi
Bug: 144248501
Bug: 143136634

Change-Id: Ib95f68bc064fbe1475e4b25433bee6292bc005dc