Stellar: support non-native assets + custom fee

[quietbits]

A few things to note:

1. FIXED - Send amount screen doesn't work with a non-native asset. It looks like it is always using the native asset (XLM), even though it shows USDC. This affects the rest of the flow (mismatch between what we see on the screen and what is actually updating). For example, it is always showing XLM on the Ledger device.
2. FIXED - Is there a way to add asset code and asset issuer conditional fields on the Send confirmation screen? We have added those fields to stellar/TransactionConfirmFields.js but they are not showing up. This may be also caused by the issue above.
3. We used BTC fees as an example, but our use case is different. We're open to suggestions on how to improve it.
4. We have been developing only on Android, so hopefully there will be no issues with iOS (we have no way of testing/developing for iOS).

Feedback is very welcome! 🙏