FR: Add ability to change app icon for OP-SEC

[tohrxyz]

problem

with the ongoing problems around self-custody, attacks from the states, we should prepare for the worst.

i think that this small and easy feature could help disguise the phoenix wallet as something different.

proposal

enable changing the app icon in the settings, so that user can change it to icon of a calculator app, notes or weather. this can be really helpful and good for maintaining good opsec.