Only register immediate material expansion panels in a material expansion panel set.

Fixes #318

See issue:
#318

PiperOrigin-RevId: 209278799

Author: phantomesse

lib/material_expansionpanel/material_expansionpanel_set.dart

@@ -17,7 +17,7 @@ class MaterialExpansionPanelSet implements OnDestroy {
   MaterialExpansionPanel _openPanel;
   List<MaterialExpansionPanel> _panels;
 
-  @ContentChildren(MaterialExpansionPanel)
+  @ContentChildren(MaterialExpansionPanel, descendants: false)
   set panels(List<MaterialExpansionPanel> panels) {
     _panels = panels;
     _onPanelsChange();