Use the default activity mipmap

So we can use different scales of the default activity icon, switch to
using the mipmap version of it.

Bug: 3325002
Change-Id: I3307bc8959829958e0da4ab2a9238c89ff9fdead
diff --git a/src/com/android/launcher2/IconCache.java b/src/com/android/launcher2/IconCache.java
index ae8c98a..468645b 100644
--- a/src/com/android/launcher2/IconCache.java
+++ b/src/com/android/launcher2/IconCache.java
@@ -67,7 +67,7 @@
 
     public Drawable getFullResDefaultActivityIcon() {
         return getFullResIcon(Resources.getSystem(),
-                com.android.internal.R.drawable.sym_def_app_icon);
+                com.android.internal.R.mipmap.sym_def_app_icon);
     }
 
     public Drawable getFullResIcon(Resources resources, int iconId) {