To clearly distinguish initialization method from getting tracker instance method.

And move all methods invocations to static `ParselyTracker`. Throw exception if using `ParselyTracker` methods without prior `init`

As interface is no longer needed

It's `private` method, not exposed to the client so there's no point in adding this annotation. Also, I don't add it to public methods because I don't want force clients to handle exception each time they want to call SDK.

The `ParselyTracker#tearDown` method was added as singleton persisted between unit tests, causing some of them to fail. I could introduce a similar behavior using reflection, but I believe `internal tearDown` method is cleaner and is not problematic as not exposed to the client

To align with other exception and move responsibility from `ParselyTracker`

And `ParselyTracker#sharedInstance` method. Allows consumer clients to
mock the `ParselyTracker` interface in their tests.

Instead of internal class `ParselyTrackerInternal`

So they're easily visible for consumers