fix(material-experimental/slider): incorrectly rendering extra background

[crisbeto]

Currently the slider always has the mdc-slider--display-markers class which causes an extra background to be rendered on sliders without markers. Since it's incorrect, these changes only add the class when markers are supposed to be rendered.

For reference:

[jelbourn]

LGTM

[devversion]

The reason it was done like that is that the slider foundation does not recognize that the track markers class has changed. So adding it always fixed it, but I didn't realize it resulted in an extra background all the time.

We should figure out how to fix that, because fixing it like that opens up another bug where tickInterval cannot be toggled dynamically if it is initially set to 0.

[crisbeto]

Updated to also handle the scenario where the slider starts off without markers.

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