Allow java_sdk_library to specify doctags_files

When generating Javadoc the processor needs to be given information
about the doctags that are present in the source. This change allows
that information to be managed with the java_sdk_library that generates
the stubs source from which the Javadoc is generated.

Bug: 168301990
Test: Built offline-sdk-docs with and without the change and
      diffed them. The only difference was the timestamp.js
      file.
Change-Id: I4adbeb0781bc2191461fec856ffa90ea185e7434