Encrypting the database on Android is a major pain in the, well, any body part really.
For what it's worth, AquaMail has a "pin code protection" (app settings -> security), should buy you enough time for a remote wipe hopefully (if you phone is lost / stolen).
The app's database is in "internal" memory,. no access without rooting. Passwords are stored encrypted anyway.
Recent Android versions can encrypt the device's "disk" memory independently of apps, I think that's something worth looking into if you're a security conscious user.
As for PGP and related -- sorry, not there, and not near the top of my priority list.