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And moved helper function into gn_helpers.py

It was only ever in .properties format so that it
could be easily parsed by bash, but we no longer need
that.

Also fixes default_android_sdk_version to be a string.
-next builds need it to be a string.

Bug: None
Change-Id: I99541f18510de93dae4c0d68b734638564f74ee2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340673
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#796020}