Are mobile wallet app secure ? How to make them more secure ?

Hi, I wonder what part of all ETH are stored on mobile wallet apps. It's clear that it's easier to use and you're ​ready to make payment anytime without your computer on, but if smartphones are not safe to hold keys, what is the chance of having keys stored in a mobile wallet app stolen?

What could we do to strengthen security?

Do we have any software answers for that or any attempt are vain without a hardware security?

Some suggested to me that having the keys held by a secure server (like one in a bank) is the only secure way to proceed and having this server take care of making transactions with the chain is like having prepaid card or an actual bank account and banks are safe places.

But I don't feel comfortable with the idea of centralised system as I think crypto currency goal it to be decentralised and eliminate third parties. Also is bank network so secure? Perhaps more secure than some crypto exchanges, a lot were​ hacked last few months, but I can't believe they are unbreakable. It involves trust, and trusting banks is not my favourite hobby...

If anyone have some suggestions to think about all this. I'm not looking for a definitive answer but for some interesting tracks. Thanks.