yes....but did Java have all of the millions of apps that were indexed by a single entity, and more importantly made it easy for anybody to access and use?
Neither does Android. Oh, there are millions of apps, but most of them are completely uninteresting on a desktop or laptop and the rest won't run well. Oh, there will be apps, over time, but there's no huge number already available, developers are going to have to start more or less from scratch.
The index is new-ish, yes, but I still don't think it's going to provoke the sort of sea change the GGP supposes. If that were all it took, the Chrome store would already be doing it (there's also an index of apps).
No, what's really going to happen is that Microsoft is going to continue its slow, gradual slide into obscurity, unless it finds a way to create a new market for itself (which is likely, frankly, though no one knows what it'll be). Android apps on Windows may even play a role, but a small one. Phones and tablets are becoming the dominant computing platform for the masses, a platform they don't participate in meaningfully, and a combination of web apps, cross-platform toolkits (like Android, but also including Java, their own .NET, Qt and Chrome apps) and maturation of free/open source offerings are breaking their stranglehold on the rest.