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We do a number of tasks in javac.py after the javac command completes:
- Move annotation processor generated files to out/Default/gen.
  - Scan all java files (including aforesaid generated ones) to
    construct .apk.jar.info files.
- Pack all .class (and other files?) into a .jar file for future build
  steps.

This CL splits out these tasks and does them in parallel, in particular
also processing java files in parallel.

Estimated incremental build savings: ~1.2s

Bug: 906803
Change-Id: Ic7a4d4e4cee998ea15f90a1c435de34e60780a3a
Reviewed-on: https://chromium-review.googlesource.com/c/1352806
Commit-Queue: Peter Wen <wnwen@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613227}