Switch global key registry to be owned by the build owner. (#74701)

This gets away from the extra static map for global key registrations
in favor of the data structures being instance properties of the build
owner. This still allows for semantically-equivalent static access through
the binding (which in turn gives access to the build owner).

This also adds a `BuildOwner.globalKeyCount` getter to get the count
of global keys associated with widgets currently in the tree.

packages/flutter/test/widgets/framework_test.dart

@@ -1500,6 +1500,20 @@ void main() {
     TestRenderObjectElement().debugFillProperties(builder);
     expect(builder.properties.any((DiagnosticsNode property) => property.name == 'renderObject' && property.value == null), isTrue);
   });
+
+  testWidgets('BuildOwner.globalKeyCount keeps track of in-use global keys', (WidgetTester tester) async {
+    final int initialCount = tester.binding.buildOwner!.globalKeyCount;
+    final GlobalKey key1 = GlobalKey();
+    final GlobalKey key2 = GlobalKey();
+    await tester.pumpWidget(Container(key: key1));
+    expect(tester.binding.buildOwner!.globalKeyCount, initialCount + 1);
+    await tester.pumpWidget(Container(key: key1, child: Container()));
+    expect(tester.binding.buildOwner!.globalKeyCount, initialCount + 1);
+    await tester.pumpWidget(Container(key: key1, child: Container(key: key2)));
+    expect(tester.binding.buildOwner!.globalKeyCount, initialCount + 2);
+    await tester.pumpWidget(Container());
+    expect(tester.binding.buildOwner!.globalKeyCount, initialCount + 0);
+  });
 }
 
 class _FakeFocusManager implements FocusManager {