I finally found an open source Android application to create image collages (combining several images/photos into a single one): Image Toolbox. The source code is available on Github, the app is installable via F-Droid
Image Toolbox's collage maker tool is available under Create -> Collage Maker. It lets you create collages from 2-10 images and has many layout options which can be switched on the fly.
You can zoom and rotate the single image's contents and move the visible image clip. Switching images with each other is no problem. The final image's aspect ratio can be changed, as well as the border radius, border color and output format.
