Swrve 6749 android easier engagement

https://swrvedev.jira.com/browse/SWRVE-6749

Reading the meta data from manifest needs to only happen once. This is done in SwrveGcmNotification.getInstance.

I also made changes to the DemoApplication to show that the when the Application is created that Swrve instance should be instantiated. This single instance is required to generate the engaged event from the SwrveGcmIntentService implicitly.

Added two Log debugs in SwrveGcmIntentService.

@Sergio-Mira 
I still think we need to add a new method to the singleton class that allows to check if there is an instance. It could happen that we get an instance that is not initialized, and would be kept on memory.