Changes:
- previously NestedGV#onTouchEvent event would call onTouchEventForResult where
we would make possible the integration of GV in CoordinatorLayout.
Since NestedGV#onTouchEvent does not anymore call onTouchEventForResult I've
moved all code in onTouchEvent and then call onTouchEventForResult only for
ACTION_DOWN events as snorp recommended.
- we now must wait for onTouchEventForResult#GeckoResult<Int> and only then
call startNestedScroll(..) since otherwise when executing that method in
BrowserToolbarBottomBehavior we would return false since the MotionEvent is
at that time unhandled.
- only dispatch startNestedScroll and dispatchNestedPreScroll if GV returned
INPUT_RESULT_HANDLED.
- Ignored "ClickableViewAccessibility" since NestedGeckoView is not intented
to be clickable, but rather the elements from the webpage displayed.

In testing I saw most of the times GeckoResult resolves in < 5ms with bigger
timeouts showing a logarithmic growth. As such I think the change should be
imperceptible to users.