There isn't a shortcut for that - I'll add it to the suggestions list, but one of the biggest problems with mobile apps is 'shortcut bloat'; the lack of a keyboard means that we have to be quite selective about what we dedicate shortcuts and screenspace to, so it isn't something we can just throw in for an update.

The quickest method for me is just to add a new layer and hide the current one. It's easy to tidy up/merge/save extra copies of the file later, and is a bit faster than going through menus. I tend to end up with 'sketchbook' paintings, which a bunch of different WIP layers.