My Android phone's unlock type is set to a password, but it only contains numbers for fast unlocking (I'm well aware that a PIN is an option, but this way presents more possibilities to an attacker). My phone's USB storage is encrypted so that I must enter my password on boot, and when I do, I am provided with a numerical-only keyboard. How does my phone know that my password only contains numbers? Does it store the password in plain text?
Your password certainly shouldn't be being stored in plain text, although without reviewing the exact software your phone is running I can't rule the possibility out. More likely, when you set the password, it checked which characters are used and set a flag (in the plain-text portion of the system where the boot code and device unlock functionality live) indicating which keyboard type was most suitable. It probably shouldn't do that, but it could reasonably be described as a convenience feature.
Alternative explanation: are you 100% sure that, when you set your password, you did it in a way that allowed setting non-numeric characters? Maybe you have a PIN by accident. Try changing your password, and see what keyboard it gives you. If it's the full keyboard that you expect, that's an argument for the "sets a flag" theory. Try adding a single letter or symbol the the password and seeing if it changes the keyboard presented when unlocking.