Add histograms to track AccessibilityNodeInfo cache usage on Android

This CL adds two new UMA histograms to track usage statistics of the
Java-side AccessibilityNodeInfo cache on Android.

We add a histogram to track the maximum number of nodes that were ever
stored in the cache during a session (updated once per atomic update).
We also add a histogram to track the number of requests the Android
Framework makes for AccessibilityNodeInfo objects that are serviced
by our cache rather than being constructed from scratch.
We add a unit test to ensure tracking is happening.

This CL has no user-facing functional changes.

AX-Relnotes: N/A
Bug: 1300602
Change-Id: Iaab9f2c128d2b42696470d429c88b3b46c649d10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3487954


Reviewed-by: Abigail Klein <abigailbklein@google.com>
Reviewed-by: Evan Liu <evliu@google.com>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/main@{#975190}