[Android] Use headingLevel for heading accessibility property

[mnayef95]

Fixes #174150

This PR updates the Android AccessibilityBridge to use the headingLevel property from SemanticsNode to determine if a node should be marked as a heading for accessibility purposes.

Previously, the IS_HEADER flag was used. Now, a node is considered a heading if headingLevel > 0. This aligns with the common accessibility pattern where heading levels (like H1, H2, etc.) define the structure.

The following changes were made:

• Modified AccessibilityBridge.java to set the heading status based on semanticsNode.headingLevel > 0 for Android API level 28 and above.
• Added headingLevel to the SemanticsNode data structure in Java and its serialization/deserialization logic.
• Updated the C++ PlatformViewAndroidDelegate to include headingLevel when serializing SemanticsNode data.
• Increased kBytesPerNode in platform_view_android_delegate.h to account for the new headingLevel field.
• Added tests in AccessibilityBridgeTest.java to verify the new heading logic:
  • Nodes with headingLevel = 0 are not headings.
  • Nodes with headingLevel > 0 are headings.

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request correctly updates the Android accessibility implementation to use the headingLevel property for determining headings, moving away from the older IS_HEADER flag. The changes are consistently applied across the Java and C++ layers, including serialization and deserialization logic. The adjustment to kBytesPerNode is correct, and the new unit tests thoroughly validate the new behavior, including edge cases. I have a couple of minor suggestions to improve code clarity and test structure, but overall this is a solid improvement.

[flutter-dashboard]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

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[chinmaygarde]

The major concern is the breaking ABI change. Left some suggestions on how to avoid it.

The requested changes have been made.

[chinmaygarde]

The embedder API change LGTM. I'll leave the rest to the @flutter/android-reviewers. Thanks!

[auto-submit]

auto label is removed for flutter/flutter/175416, Failed to enqueue flutter/flutter/175416 with HTTP 400: GraphQL mutate failed.