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This CL contains a few changes related to app bundle generation.
part of a simplification of the following large CL:
https://chromium-review.googlesource.com/c/chromium/src/+/1071657

More specifically:

- bundletool.py: Enable the python script to be imported. This
  will happen in a future CL that will add new Python build
  scripts to generate app bundles and apk sets.

- Compile resources to protobuf-based format using the new
  aapt2 binary added recently to third_party/android_build_tools/.

- Add an entry in an APK's .build_config file indicating the location
  of its final dex files, to help the bundle-generating targets
  use them later.

BUG=820459
R=agrieve@chromium.org, benmason@chromium.org, yfriedman@chromium.org

Change-Id: Ifd66f11bafe804625efba3051ecf5f7c40fcf740
Reviewed-on: https://chromium-review.googlesource.com/1120530Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572223}