I use PentiKeyboard, it can send basically all the byte sequences I'm accustomed to having available plus it has a shortcut for sending ctrl+b to tmux.

https://play.google.com/store/apps/details?id=de.software_la...

Either that or I connect a wireless physical keyboard.

Edit: The killer feature of Penti is that it is transparent and allows you to put the 'buttons' where it is convenient to place the fingertips. Unlike regular software keyboards which hide half the screen and have 'buttons' that are pretty much thumbs-only. Since I code a lot I'm not particularly keen on mainstream next-word-guessing either.