[Feature] Add a option in the Settings -> General section to make fiat displayed by default

[WillXUMM]

While Xumm is not a portfolio tracker, it might be a nice feature to have a setting in the Settings -> General section to make fiat displayed by default when launching Xumm.

So instead of quantity of XRP, it would show the fiat value. (And when you tap on it is shows the XRP quantity.)

So it works the same as it currently does, but in reverse.

I think that there are people just want to see the value of their XRP in a quick and reliable way, so they launch Xumm. This feature would save them an extra step. (Having to tap the XRP amount.)