fix(select): open method triggered twice on click

[crisbeto]

Fixes the open method being triggered both by a click handler on the select trigger and the onContainerClick from the MatFormField integration.

[mmalerba]

I think this is breaking tests because people are simulating clicks on the trigger, and the simulated clicks don't bubble. I also realized that people technically should be able to use the mat-select outside of the mat-form-field if they want. Maybe instead we can just check the target of the event in onContainerClick and ignore it if it was the trigger element?

[crisbeto]

Reworked the PR to check internally for the _panelOpen flag @mmalerba.

[josephperrott]

@crisbeto Can you rebase and we can try to get this one in?

[crisbeto]

Rebased.

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