[demos] Move to subdir

This was supposed to go in the previous CL, but patchsets are hard.

Change-Id: I9c479e547bc3f206bb2225ca54b25c52b9483960
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/298407
Reviewed-by: Kevin Lubick <kjlubick@google.com>
2 files changed