Add try-with-resource support to TraceSection

[0xZOne]

This PR adds a new way to define scoped trace events in Java, using the try-with-resource construct. This makes code simpler to read, and less error-prone.

Before:

try {
  TraceSection.begin("name");
  // ....
} finally {
  TraceSection.end();
}

Now:

try (TraceSection e = TraceSection.scoped("name")) {
  // ...
}

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 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.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

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[Hixie]

test-exempt: code refactor with no semantic change

[gaaclarke]

lgtm!