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 
Fair point, we need differentiate init of SwrveSDK and sessionStart. The fact that they are linked together makes all my changes redundant.