The issue here is that a window can seemingly have child windows. For instance, the Facebook activity triggers a package packageinstaller, but the aura Window does not change. In this scenario, we're left with an active window (and so a current task id), but the actual contents belong to a different package.

The root cause of this is that a task can have multiple activities (otherwise known as the back stack). These activities can be anything (same package name, different package name), but the active task id won't change since all the activities belong to the same task.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/783dc7b57549781f1ed08a5a946f5f5c5ebf224c

commit 783dc7b57549781f1ed08a5a946f5f5c5ebf224c
Author: David Tseng <dtseng@chromium.org>
Date: Fri Oct 06 00:01:30 2017

Plumb through task id from Arc accessibility

This change replaces most of the mappings required for package name to
tree source translations. It results in a lot of cleanup namely:
- removal of all package name specifics
- removal of various observations for task id creation/activeness
- removal of current task id state
- removal of fallback tree

work (e.g. actions, browsing, etc).

Test: open many apps. Verify touch explore, ChromeVox interactions
Bug:  768532 
Change-Id: Ic6f679fc8bb3f649334a29f98e86a34b52e5cfbc
Reviewed-on: https://chromium-review.googlesource.com/688023
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Yuki Awano <yawano@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506923}
[modify] https://crrev.com/783dc7b57549781f1ed08a5a946f5f5c5ebf224c/chrome/browser/chromeos/arc/accessibility/arc_accessibility_helper_bridge.cc
[modify] https://crrev.com/783dc7b57549781f1ed08a5a946f5f5c5ebf224c/chrome/browser/chromeos/arc/accessibility/arc_accessibility_helper_bridge.h
[modify] https://crrev.com/783dc7b57549781f1ed08a5a946f5f5c5ebf224c/chrome/browser/chromeos/arc/accessibility/arc_accessibility_helper_bridge_unittest.cc
[modify] https://crrev.com/783dc7b57549781f1ed08a5a946f5f5c5ebf224c/chrome/browser/chromeos/arc/accessibility/ax_tree_source_arc.cc
[modify] https://crrev.com/783dc7b57549781f1ed08a5a946f5f5c5ebf224c/chrome/browser/chromeos/arc/accessibility/ax_tree_source_arc.h
[modify] https://crrev.com/783dc7b57549781f1ed08a5a946f5f5c5ebf224c/components/arc/common/accessibility_helper.mojom