Add AccessibilityAction and AccessibilityEvent API for drag & drop

go/a11yDragAndDrop

Drag & drop in the system has zero accessibility support.
Add actions to dragging, cancelingl, and dropingp
Add WINDOW_CONTENT_CHANGED subtype events for the start of a drag,
the cancellation of a drag, and a drop. Other changes, like
when a11y actions change, will be denoted by
CONTENT_CHANGE_TYPE_UNDEFINED

Test: builds
Bug: 26871588
Change-Id: Ie4eeb54dd612fd92c107a267e884498656c1fac8