You don't have a choice for restore. You either use the keyboard on your PC or your phone.
I know. I still don't yet have a Trezor for this reason. Maybe the Armory plugin will be written in a way that allows offline seed restore/create
Although I have a feeling that an Android 5.1 NOT rooted might even be safer than an Online Linux PC, the other apps not being able to access the other apps it's storage space and all this, BUT I'm no security expert... Would even like an answer to this very question.
I wouldn't take that feature especially seriously TBH. Maybe if the SELinux enforcement was enabled by default, but that's not going to happen. If you're thinking "well, I'll just turn this SELinux thing on then", good luck with that.
Restoring seeds always seems to be the weakest link in the whole system. If the "Trezor Tools" app implemented "advanced word entry" by inputting the seed words using the pin pad it would be impossible to steal the seed instead to too difficult to brute force the order. The user would have to look up each seed word and find the associated number and enter it using the randomized pin pad. The host computer would not be able to decipher the word being entered just like the PIN code.
And that's the only other part of the Trezor system I dislike, although it's not so bad if you can use the trezor-python library commands from an offline system. Sure, it would make the seed restore operation less user friendly, but who needs that with a function you're unlikely to use all that often? (or at least not as often as other functions like signing transactions)