Fix the switch to match the Material Design spec.

Our track and head were slightly off in size.
These now exactly match the sizes in
https://material.google.com/components/selection-controls.html#selection-controls-switch
We took the liberty of assuming the head overlaps the edge
of the track by 2 pixels (instead of the head and the track ending at the same point)
which the provided designs were conflicted about.

Huge thanks to @DaveShuckerow for noticing this issue!

@abarth

packages/flutter/lib/src/material/switch.dart

@@ -158,7 +158,7 @@ class _SwitchRenderObjectWidget extends LeafRenderObjectWidget {
 }
 
 const double _kTrackHeight = 14.0;
-const double _kTrackWidth = 29.0;
+const double _kTrackWidth = 33.0;
 const double _kTrackRadius = _kTrackHeight / 2.0;
 const double _kThumbRadius = 10.0;
 const double _kSwitchWidth = _kTrackWidth - 2 * _kTrackRadius + 2 * kRadialReactionRadius;
@@ -355,7 +355,7 @@ class _RenderSwitch extends RenderToggleable {
     }
 
     // The thumb contracts slightly during the animation
-    double inset = 2.0 - (currentPosition - 0.5).abs() * 2.0;
+    double inset = 1.0 - (currentPosition - 0.5).abs() * 2.0;
     double radius = _kThumbRadius - inset;
     Rect thumbRect = new Rect.fromLTRB(thumbPosition.x + offset.dx - radius,
                                        thumbPosition.y + offset.dy - radius,