Update Stocks.apk build tech

Instead of having a one-off //sky/apk/stocks, we now use the |sky_app| GN
template to create Stocks.apk directly from //sky/sdk/example/stocks.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1211883004.

engine/src/flutter/BUILD.gn

@@ -18,10 +18,6 @@ group("sky") {
     ":sky_apk",
   ]
 
-  if (is_android) {
-    deps += [ "//sky/apk/stocks" ]
-  }
-
   if (!is_android) {
     deps += [ "//third_party/mesa:osmesa" ]
   }