Cleanup events related to material splashes

This CL is a warmup for using a more sophisticated gesture disambiguation.

1) Use gesturetap instead of click. We should probably remove click events
   because folks should use gesturetap to integrate with the gesture system.

2) Handle the case where you swipe the drawer during an animation. Previously
   we had an assert which triggered in some multitouch scenarios. We'll
   eventually move this over to gestureswipe.

3) Remove an extra container for ink splashes. There's no need to group all the
   ink splashes in a container. They can all just be children of the Material
   component itself. This structure is left over from when Material was a base
   class.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1013713005

examples/stocks-fn/stocksapp.dart

@@ -109,7 +109,7 @@ class StocksApp extends App {
         new Icon(key: 'menu', style: _iconStyle,
             size: 24,
             type: 'navigation/menu_white')
-          ..events.listen('click', _drawerAnimation.toggle),
+          ..events.listen('gesturetap', _drawerAnimation.toggle),
         new Container(
           style: _titleStyle,
           children: [title]
@@ -117,7 +117,7 @@ class StocksApp extends App {
         new Icon(key: 'search', style: _iconStyle,
             size: 24,
             type: 'action/search_white')
-          ..events.listen('click', _handleSearchClick),
+          ..events.listen('gesturetap', _handleSearchClick),
         new Icon(key: 'more_white', style: _iconStyle,
             size: 24,
             type: 'navigation/more_vert_white')