When approving a payment transaction, display currency-code in both HEX and ASCII about app-xrp

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Displaying 160-bit HEX strings is a suboptimal user-experience

Hex strings that can be decoded to all ASCII are supposed to be decoded to ASCII. I see a bug on line https://github.com/LedgerHQ/app-xrp/blob/master/src/xrp/amount.c#L176, that's supposed to be negated. If "XRP" is in the hex string, we should proceed to the else-case and display as hex.

Earlier versions of this firmware properly decoded all ASCII hex strings, e.g. 5553445400000000000000000000000000000000 to USDT. The decoder for all ASCII can be seen here: https://github.com/LedgerHQ/app-xrp/blame/master/src/xrp/strings.c#L21 and it behaves as expected for other decoding of e.g. MemoData.

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Original Towo Labs code to the left: https://github.com/TowoLabs/ledger-app-xrp/blob/master/src/xrp/format/amount.c#L64

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Ping @TamtamHero, you probably wanna look at the above diff. since it inverted the intended behaviour.

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This is still an issue in production. The expected behaviour has changed. Pinging for higher visibility @TamtamHero @fbeutin-ledger @tjoly-ledger @agrojean-ledger @btchip