Create experimental JNI implementation for Android accessibility

Start an experiment that takes a different approach to the JNI
calls for the Android accessibility code. Instead of passing a
large object over the boundary, pass an id and use the existing
cache in the Java-side code to reference the object. This
experiment may not be pushed beyond Beta depending on how the
initial data looks.

AX-Relnotes: N/A
Bug: N/A
Change-Id: I44ceb6e7ade9c03fd221b8d3369d51bbf2ee8744
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5253290
Reviewed-by: David Tseng <dtseng@chromium.org>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/main@{#1255111}

• content/browser/accessibility/web_contents_accessibility_android.cc[diff]
• content/browser/accessibility/web_contents_accessibility_android.h[diff]
• content/browser/android/content_feature_map.cc[diff]
• content/public/android/java/src/org/chromium/content/browser/accessibility/AccessibilityNodeInfoBuilder.java[diff]
• content/public/android/java/src/org/chromium/content/browser/accessibility/WebContentsAccessibilityImpl.java[diff]
• content/public/android/java/src/org/chromium/content_public/browser/ContentFeatureList.java[diff]
• content/public/common/content_features.cc[diff]
• content/public/common/content_features.h[diff]
• testing/variations/fieldtrial_testing_config.json[diff]