fix(panel): Element reference error

[bradrich]

Actual Behavior:

• What is the issue? * In PR feat(mdPanel): Wrapper and Panel elements referenced in the MdPanelRef #9231, I made the panelContainer and panelEl references public within the API. When I went through and added the deprecation notices to the addClass, removeClass, and toggleClass methods, I overlooked the fact that panelContainer and panelEl were public and I left them private. Tests didn't fail because of the defined exceptions.
• What is the expected behavior? panelContainer and panelEl need to be public everywhere.

CodePen (or steps to reproduce the issue): *

• CodePen Demo which shows your issue: N/A
• Details: N/A

Angular Versions: *

• Angular Version: Latest
• Angular Material Version: Latest

Additional Information:

• Browser Type: * All
• Browser Version: * All
• OS: * All
• Stack Traces: N/A

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