Prevent LayoutBuilder from rebuilding more than once (#147856)

Fixes flutter/flutter#146379: introduces `Element.buildScope` which `BuildOwner.buildScope` uses to identify subtrees that need skipping (those with different `BuildScope`s). If `Element.update` calls `updateChild` then dirty children will still be rebuilt regardless of their build scopes. 

This also introduces `LayoutBuilder.applyDoubleRebuildFix` migration flag which should only live for a week or less. 

Caveats: 

`LayoutBuilder`'s render object calls `markNeedsLayout` if a descendant Element is dirty. Since `markNeedsLayout` also implies `markNeedsPaint`, the render object is going to be very repaint/relayout-happy.

Tests: 

Presubmits with the migration flag set to true: https://github.com/flutter/flutter/pull/147856/checks?check_run_id=24629865893

packages/flutter/lib/src/rendering/object.dart

@@ -1999,8 +1999,7 @@ abstract class RenderObject with DiagnosticableTreeMixin implements HitTestTarge
         }
 
         if (!activeLayoutRoot._debugMutationsLocked) {
-          final RenderObject? p = activeLayoutRoot.debugLayoutParent;
-          activeLayoutRoot = p is RenderObject ? p : null;
+          activeLayoutRoot = activeLayoutRoot.debugLayoutParent;
         } else {
           // activeLayoutRoot found.
           break;
@@ -3004,7 +3003,7 @@ abstract class RenderObject with DiagnosticableTreeMixin implements HitTestTarge
         owner!._nodesNeedingPaint.add(this);
         owner!.requestVisualUpdate();
       }
-    } else if (parent is RenderObject) {
+    } else if (parent != null) {
       parent!.markNeedsPaint();
     } else {
       assert(() {
@@ -3020,9 +3019,7 @@ abstract class RenderObject with DiagnosticableTreeMixin implements HitTestTarge
       //
       // Trees rooted at a RenderView do not go through this
       // code path because RenderViews are repaint boundaries.
-      if (owner != null) {
-        owner!.requestVisualUpdate();
-      }
+      owner?.requestVisualUpdate();
     }
   }