This is not complete gesture support by far, but it's a start.
MojoShell and Chrome use a C++ GestureDetector, this code attempts
to use the Android (Java) GestureDetector instead.

We probably should not be sending gesturetap until we've decided
it's not a scroll, etc, but this implementation does not go that far.

I had to fix a bug whereby we were assuming the InputEvent.time_stamp
was in TimeDelta's internal format, which was wrong.  When we get
time_stamp from Android its in ms since boot.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/969493002