Migrate third_party/expat dependency to googlecode

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third_party/expat was recently migrated from flutter/buildroot to our
googlesource repo at https://flutter.googlesource.com/third_party/expat
in [1] and deleted from the buildroot in [2]. This rolls the buildroot
and adds the new upstream dependency.

[1] https://flutter-review.googlesource.com/c/third_party/expat/+/12700
[2] flutter/buildroot#445

Bug: flutter/flutter#39532

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.
• The reviewer has submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

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This is dependent on flutter/buildroot#445 landing (at which point the buildroot SHA will need to be updated).

[cbracken]

Now that flutter/buildroot#445 has landed, this has been updated.