Change postBuildExtras to use cordova provided list

This fixes the problem where multiple cordova plugins attempting to use the postBuildExtras in the gradle script will simply clobber each other. The cordova provided gradle template provides a defined list for plugins to add their post build extras to so they can all play nicely together.

https://github.com/apache/cordova-android/blob/73edf4de7b751ecc103e9fad3c6d71c149c9118e/bin/templates/project/app/build.gradle#L90

This fixes the problems outlined in: 
meteor/meteor#7600
Meteor-Community-Packages/raix-push#334

src/android/build-extras.gradle

@@ -16,7 +16,7 @@
        specific language governing permissions and limitations
        under the License.
  */
-ext.postBuildExtras = {
+ext.cdvPluginPostBuildExtras << {
     def inAssetsDir = file("assets")
     def outAssetsDir = inAssetsDir
     def outFile = new File(outAssetsDir, "cdvasset.manifest")