First of all, as you may know AnkiDroid is a project independent from Anki. We don't really have any ongoing costs that require donations.
Bounties would be appreciated though if there are specific features that you want.
Daimen (the developer of all the other Anki clients) does incur ongoing costs for AnkiWeb etc, but he's had to temporarily stop the donation system after moving country, meaning currently the only way to contribute to him is to buy the iOS software. You can probably still purchase the app with iTunes even if you don't have an iOS device though, and simply not use it, or send it as a gift to a friend or something.