You may consider naming your views with ids that are prefixed with the type of view i.e tipLabel => tvTipLabel makes it easier to understand the code in the java.
You could consider error handling, if you enter no amount and hit the tip buttons, its possible the app would raise an exception
The way you handled the onclick listener was very clean, adding it in the xml and then using a single method which determines percentage tip based on id.
Nice work overall, let us know if you have any questions. See you in class tomorrow.
My app is a complete, please review. /cc @nesquena @timothy1ee