Swaps: fix initial input native currency value, light mode coin icon placeholder

[christianbaroni]

Fixes APP-1605

What changed (plus any additional context for devs)

• Fixes the input native currency value being 0 initially, due to separators in the input value added by useGrouping
• Fixes the light mode swap coin icon placeholder
• Resets the slider position to 50% when the input asset changes

Screen recordings / screenshots

What to test

[linear[bot]]

APP-1605 Regression: input asset native amount starts out at 0 ($0.00), then populates