OK, after some head-bashing I've realised there's a Linux-specific (Android being based on Linux) mistake being made here - when you type `/src/dir` the initial `/` implies that you're looking in the filesystem root, mounted at `/`. In this case, however, you're probably referring to the current directory, which will likely be in `/sdcard/` since you are not rooted. Simply omit the first slash and you're good to go (personally tested).

    find src/dir -type f -exec mv --backup=numbered -t dst/dir {} +