Fix backward compatibility in LeakCanary 3.0 alpha 1

[pyricau]

• DeletableObjectReporter isn't a fun interface so custom code that was passing a lambda to watchers now has to pass object : DeletableObjectReporter. This should be an easy fix, just add some fun.
• RootViewWatcher now has a required watchDismissedDialogs field. Let's provide a default value.
• DeletableObjectReporter must return a TrackedObjectReachability. That's not backward compatible. Not easy to implement when delegating to AppWatcher.objectWatcher.expectWeaklyReachable(target, reason), and even if switching to expectDeletionFor we need to handle the case where we actually want to skip and not delegate. Best seems to provide to versions of the method, one of which doesn't return a TrackedObjectReachability. That'll make things easier for the ReachabilityWatcher implementation as well.