Verify the existence of debug symbols in gradle_plugin_bundle_test (#162645)

A small piece of https://github.com/flutter/flutter/pull/162464 that I
wanted to land anyways.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
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[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
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Co-authored-by: Gray Mackall <mackall@google.com>

dev/devicelab/bin/tasks/gradle_plugin_bundle_test.dart

@@ -39,6 +39,8 @@ Future<void> main() async {
           'base/lib/arm64-v8a/libflutter.so',
           'base/lib/armeabi-v7a/libapp.so',
           'base/lib/armeabi-v7a/libflutter.so',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/arm64-v8a/libflutter.so.sym',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/armeabi-v7a/libflutter.so.sym',
         ], await getFilesInAppBundle(releaseBundle));
       });
 
@@ -72,6 +74,8 @@ Future<void> main() async {
           'base/lib/arm64-v8a/libflutter.so',
           'base/lib/armeabi-v7a/libapp.so',
           'base/lib/armeabi-v7a/libflutter.so',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/arm64-v8a/libflutter.so.sym',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/armeabi-v7a/libflutter.so.sym',
         ], await getFilesInAppBundle(bundleFromGradlePath));
 
         section('Build app bundle using the flutter tool - flavor: flavor_underscore');
@@ -103,6 +107,8 @@ Future<void> main() async {
           'base/lib/arm64-v8a/libflutter.so',
           'base/lib/armeabi-v7a/libapp.so',
           'base/lib/armeabi-v7a/libflutter.so',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/arm64-v8a/libflutter.so.sym',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/armeabi-v7a/libflutter.so.sym',
         ], await getFilesInAppBundle(flavorUnderscoreBundlePath));
 
         section('Build app bundle using the flutter tool - flavor: production');
@@ -134,6 +140,8 @@ Future<void> main() async {
           'base/lib/arm64-v8a/libflutter.so',
           'base/lib/armeabi-v7a/libapp.so',
           'base/lib/armeabi-v7a/libflutter.so',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/arm64-v8a/libflutter.so.sym',
+          'BUNDLE-METADATA/com.android.tools.build.debugsymbols/armeabi-v7a/libflutter.so.sym',
         ], await getFilesInAppBundle(productionBundlePath));
       });