Swaps: BTC selected by default (starting state logic)

[lion-dev]

A logic to define which assets should be selected by default could look like:

• User comes from btc layer, btc is selected by default (send input), no selection on receive input.
• User comes from other layer and layer balance is 0, btc is selected by default (send input), other layer is selected by default on receive
• User comes from other layer and layer balance is NOT 0, layer default token is selected by default (send input), no selection on receive.