fix(autocomplete): placeholder should float while panel is open

[kara]

Ready for review

r: @jelbourn

[andrewseguin]

Looks good to me

[gedclack]

Hi, I am new in github and also new to angular,
I met this situation where Input's placeholder still floats even after I set floatPlaceholder="never" in the <md-input-container>, is this intentional according to this commit ?

[dahaupt]

@gedclack If you have a question you can ask it in https://gitter.im/angular/material2 and in case you think it's an issue in the library, you can open an issue.

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