Change the default docs build to use module stubs

The two are almost identical, and the target using sources as input
can be deleted.

The diffs that remain are minor and as far as I can tell OK:
- A few constants are removed from the metadata.zip. It looks to
  me like metalava was previously including @SystemApi constants in the
  public SDK's metadata. This bug is now worked around by only feeding
  metalava the public APIs.
- There are some very minor diffs in the docs of a few APIs.

Bug: 186197911
Test: build sdk before/after and diff
Change-Id: Iac7de64f353bc1467fc4b249e531f034ef702d51