Finish main activity after navigating to AppDetails activity.

This is what used to happen before the recent refactor to the search UI. Finishing the main activity means that it never comes back to handle the "View this app" intent again. Instead, the main Activity just becomes an entry point for redirecting the UI to the correct place.

Incidentally, I don't particularly like the current solution even though it should work, and hope to come up with a better one in the future. The reason is because the behaviour of some methods (onCreate() + handleIntent()) is what defines the behaviour of other methods (checkForAddRepoIntent()). This means it is hard to reason about the state of the activity at any point in time. Developers need to be careful when making changes to the onCreate() method because modifying it has unintended consequences. That is what caused the problem in issue #541 (closed).

Fixes #541 (closed).