If you plan to use Ubuntu Touch OS by UBports then you’ll most certanly be able to run Android apps on Librem5 since UBports is developing their project Anbox to do just that.
I asume this will be possible from day one since afaik UBports is getting dev kits from Purism to make everything ready for the launch.
You can try Anbox from www.anbox.io on your Linux desktop or Ubuntu Touch supported phones (with Mediatek processors for now)