Fix broken activation of the selected view in accessibility mode.

We were using an approximation to determine where to send a pair of down
and up events to click on the view that has accessibility focus. We were
doing reverse computation to figuring out which portion of the view is
not covered by interactive views and get a point in this region. However,
determining whether a view is interactive is not feasible in general since
for example may override onTouchEvent. This results in views not being
activated or which is worse wrong views being activated.

This change swithes to a new approach to activate views in accessibility
mode which is guaranteed to always work except the very rare case of a
view that overrides dispatchTouchEvent (which developers shouldn't be
doing). The new approach is to flag the down and up events pair sent
by the touch explorer as targeting the accessibility focused view. Such
events are dispatched such that views predecessors of the accessibility
focus do not handle them guaranteeing that these events reach the accessibiliy
focused view. Once the accessibiliy focused view gets such an event it clears
the flag and the event is dispatched following the normal event dispatch
semantics.

The new approach is semantically equivalent to requesting the view to perform
a click accessiblitiy action but is more generic as it is not affected by
views not implementing click action support correctly.

bug:18986806
bug:18889611

Change-Id: Id4b7b886c9fd34f7eb11e606636d8e3bab122869