Are there any online/standalone editors or template collections for designing Android application?
It would be difficult to find a site with templates for android apps, as each app has different functions. You don't specify what platform you are using for developing them, so I'm assuming you will use HTML. In that case, any template for 320px width or responsive template that includes mobile will work.
If you are creating your app from zero, I'd recommend you start with a UI kit for 'inspiration'. Some are:
I would suggest for the actual designing you use Photoshop or similar software, then there are a couple of excellent kits which give you the basics.
Googles own set of downloads which covers pretty much every UI element 4.2.1 has.
Android UI Design Kit gives you a collection of "building blocks" which you can edit to suit your design.
Crystal Design Kit is neither for iOS or Android and can easily be adapted to either
Lookamore on the other hand is specifically designed for Android.
For .sketch there is this Android Kit
For prototyping there are a few online tools, although they are very limited unless you are willing to subscribe. Proto.io is widely considered to be an excellent mobile prototyping tool and Fluid UI is also well worth a look.
Also it's worth noting that the most common screen size these days is actually roughly 1280 by 720px and some phones, such as my Sony Xperia Z, have a full HD screen. So it's best to work large and scale down.
And of course you can, as mentioned by Yisela, adapt a web UI kit to suit your needs.