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 
Unfortunately the SDK is not designed to do this. If put in the Application onCreate, it will send a session start when the app comes up (even with no UI). The metrics would be wrong as there might have not been user interaction with the app's UI.

This will be possible once we redesign the SDK interface, allowing to init the SDK here but only add the automatic data in each Activity's onCreate.