Gradle: Use cdvPluginPostBuildExtras instead of ext.postBuildExtras

[merrygobyebye]

Cordova Plugins should use cdvPluginPostBuildExtras (https://github.com/apache/cordova-android/blob/e9750b5cabe70b1fc9abfad28fd1239d9b9415c2/bin/templates/project/app/build.gradle#L90) to have code run at the end of the build gradle script instead of ext.postBuildExtras (https://github.com/apache/cordova-android/blob/e9750b5cabe70b1fc9abfad28fd1239d9b9415c2/bin/templates/project/app/build.gradle#L321). When multiple plugins use ext.postBuildExtras, each plugin will overwrite the previous plugin's assignment of it. The offending lines of code in this plugin are here: https://github.com/blakgeek/cordova-plugin-flurryanalytics/blob/f67d76bfad8c77603b93818ce82bb622b67c18a4/src/android/plugin.gradle#L1-L8