Populate a11y action in event throttle of ViewRootImpl

In ViewRootImpl#SendWindowContentChangedAccessibilityEvent throttles
a11y events of TYPE_WINDOW_CONTENT_CHANGED, and may merge multiple
events into one.

This changes adds merging logic of AccessibilityEvent#getAction.

Bug: 263246622
Test: CtsAccessibilityServiceTestCases
Change-Id: I3b80a26520f1ec6587deb7991b9d6287e3067131