Add third_party/android_platform and tools/python from the Chromium git
to satisfy dependencies for the build/android/tombstones.py script.

Bug: angleproject:5933
Change-Id: Ie19eec46c8cab208cde5064118ae655303e12fa7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2878848Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Lubosz Sarnecki <lubosz.sarnecki@collabora.com>