Improve experience on low end devices

[MrStahlfelge]

On very low end devices (that might be used for cold wallet) decrypting and encrypting mnemonic takes its time, on a Moto G 4G it is about 5 seconds.
When password protection is used, this is done on UI thread and the password request screen "hangs". Situation should be improved and password screen should show a progress bar as well es encryption/decryption should be done on a background thread.
Biometrics should be checked how the behaviour looks like.

No problem on iOS due to devices being fast.