Use drawable_xxhdpi instead of 2x_web

This makes the icons look nicer on the Nexus 5

I briefly played with trying to figure out which drawable_*
to use dynamically, but I couldn't seem to write a reasonable
solution which worked on both Desktop and my Nexus 5, leading
me to believe I didn't understand how it should work and just
went with xxhdpi for now.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1186633003.

BUILD.gn

@@ -36,11 +36,13 @@ group("sky_apk") {
     "//mojo/dart/mojom",
     "//mojo/public/dart:mojo",
     "//sky/sdk",
-    "//sky/services/intents",
     "//third_party/dart-pkg",
   ]
 
   if (is_android) {
-    deps += [ "//sky/apk/demo" ]
+    deps += [
+      "//sky/apk/demo",
+      "//sky/services/intents",
+    ]
   }
 }

sdk/lib/framework/widgets/icon.dart

@@ -26,9 +26,12 @@ class Icon extends Component {
       category = parts[0];
       subtype = parts[1];
     }
+    // TODO(eseidel): This clearly isn't correct.  Not sure what would be.
+    // Should we use the ios images on ios?
+    String density = 'drawable-xxhdpi';
     return new Image(
       size: new Size(size.toDouble(), size.toDouble()),
-      src: '${kAssetBase}/${category}/2x_web/ic_${subtype}_${size}dp.png'
+      src: '${kAssetBase}/${category}/${density}/ic_${subtype}_${size}dp.png'
     );
   }