[Expansion] Standalone panel inconsistently overrides margin when collapsed vs expanded

[willshowell]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Margin set on a standalone expansion panel should not be overridden when collapsed. Alternatively, I would expect it to be overridden consistently in collapsed and expanded states.

What is the current behavior?

When collapsed, the panel has an margin of 0 applied via inline style. When expanded, no inline styles exist.

For reference:

    trigger('displayMode', [
      state('collapsed', style({margin: '0'})),
      state('default', style({margin: '16px 0'})),
      state('flat', style({margin: '0'})),
      transition(...)
    ]

  /** Gets the panel's display mode. */
  _getDisplayMode(): MdAccordionDisplayMode | MdExpansionPanelState {
    if (!this.expanded) {
      return this._getExpandedState();
    }
    if (this.accordion) {
      return this.accordion.displayMode;
    }
    return this._getExpandedState();
  }

  /** Gets the expanded state string. */
  _getExpandedState(): MdExpansionPanelState {
    return this.expanded ? 'expanded' : 'collapsed';
  }

As you can see, there is no handling of the state being 'expanded'

What are the steps to reproduce?

http://plnkr.co/edit/Oscu0l2jPi7W2HGmh5yv?p=preview

What is the use-case or motivation for changing an existing behavior?

Obviously I could wrap it in a container and add a margin to that, but it seems that inconsistent behavior in different states is undesirable.

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