Ensure touch explorer and magnifier get a well-formed event stream.

We use an input filter to manipulate the event stream in accessibility
mode. Some input events, i.e. touch and hover events, are delivered
to a touch explorer, if touch exploration is enabled, and to a magnifier,
if screen magnification is enabled. It is possible that at the moment
each of these features is enabled we are in the middle of a touch or
hover gesture. The touch explorer and screen magnifier expect to receive
an event stream that starts with an event that denotes the stream start.
This change ensures that hover or touch events are dispatched to the
touch explorer and the magnifier only after the start of the first
well-formed hover or touch sequence.

Change-Id: I8cd0ad8e1844c59fd55cf1dfacfb79af6a8916df