Output Javadoc comments for public API

When generating stubs source for contributions to the public API keep
the Javadoc comments from the input source so that the stubs source can
be used as input to the metalava invocation that creates the stubs from
which the public Javadoc is generated.

This is needed to fix an issue with conscrypt's contributions to the
public API, i.e. SSLEngines and SSLSockets where the Javadoc comments
were not present in the generated Javadoc.

Test: Built offline-sdk-docs with and without the change and
      diffed them. The only difference was the timestamp.js
      file and they now included proper documentation for the
      SSLEngines/SSLSockets classes.
Bug: 169162018
Change-Id: I1b50cff3b91766ce261b2f8e798b77ebc6faac70