Описание тега android-application-class
Extending Android's Application class can be very useful to maintain global state, initialise a module, and so on, before any other part of your code runs.
Base class for maintaining global application state. You can provide your own implementation by creating a subclass and specifying the fully-qualified name of this subclass as the
"android:name"attribute in your AndroidManifest.xml's<application>tag. The Application class, or your subclass of the Application class, is instantiated before any other class when the process for your application/package is created.