Refine `Toggle` dimensions to follow Figma spec

[chrisbobbe]

We plan to ship "Invisible mode" with a toggle switch that doesn't quite match the Figma. Quoting the PR:

#1631

Some known differences from the Figma, which won't be hard to fix but require upstream changes, so not totally trivial:

• When the switch is on:

  • The switch should be 28px by 48px, not 36px by 56px.
  • The "thumb" (white circle with checkmark) should have radius 10px, not 12px.

• When the switch is off:

  • The switch should be 26px by 46px, not 34px by 54px.
  • The "thumb" should have radius 7px, not 8px.

This issue is to resolve those differences.

Implementation

The upstream issue is flutter/flutter#131478 ; we should investigate and send a PR, then use the exposed configuration to resolve the TODO in the Toggle code.