Support GCM case

* When using GCM you need to define custom permission, in this design put those settings in separated AndroidManifest.xml
* In this example we're using different package name & maps API keys in flavours, so the origin flavour won't have related resource files. Since the manifest.srcFile is read-only in sourceSets, we use the origin flavour source directory as "bridge" to connect flavour+buildtype combination with different AndroidManifest.xml