Ability to imperatively float md-input placeholder

[kara]

Currently there is a floatingPlaceholder property on md-input-container that controls whether or not you'd like a floating placeholder at all. This property effectively disables the floating placeholder.

However, it's not meant to be an imperative way to toggle the floating placeholder on and off. If you try to set floatingPlaceholder to true, it doesn't actual float it. It will still be gated on the two conditions that the value must not be empty or the input must be focused.

We should add a way to actually toggle the placeholder, not just disable it. The motivation for this change is the autocomplete. Sometimes the text field will be empty and unfocused (e.g. if you are keying down through the option list), but the placeholder shouldn't reset.

cc @mmalerba

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