[TextField] Prefix color, spacing incorrect

[ry5n]

Expected:

• Prefix should remain color(ink, lighter) regardless of field state
• Should be 8px space between prefix and field value

Actual:

• Prefix darkens when field value entered, making it unclear that the prefix is not part of the value. Also is distracting when the prefix is an icon.
• Not enough space between prefix and value.

[ry5n]

Closed by #1597