great app but is there a way to encrypt the wallet ?
Is it not a problem if the wallet in the android application is not encrypted ?
Currently you can't. It should not be a problem because Android already isolates your app private files against all other non-root apps. So as long as you're not running anything as root you should be safe.
Alright thank you

But I use root apps (ES Explorer for example).
So you better have reasons to trust these apps very well.
Are there plans to implement this feature ?
It's planned, but there is still some work to do, and its not my top priority. One reason is it won't help in cases like yours: If an app has got root, it can probably just capture keypresses or trick you into signing their transaction. Encryption just puts the bar a little bit higher for a successful attack, but its far from a silver bullet.