[Improvement] Don't "double-calculate" the same currency

[WietseWind]

Here's an account with 19k EUR in IOU. The default currency selected in the wallet is EUR. Meaning we're looking at an IOU representation of the native asset selected in the wallet.

Now when the Account Worth xApp is opened:

1. It takes the IOU value
2. Takes the order book and checks what XRP value the IOU has
3. Taks the XRP value and turns it into fiat value

This then results in a lower EUR value, because there's slippage (lack of liquidity)

However, if my native asset is EUR, and the IOU is EUR, I won't end up with a lower amount of EUR when looking at net worth: it should stay unconverted.

Imo, there should be one addition to the current calculations:

• If IOU (XRPL asset code) equals user native asset code, then don't convert but use value as-is