feat(slider): add focus and blur methods do MatSlider class

[julianobrasil]

Does it need any tests?

Also there's a little side effect: if the thumb label is already visible and you try to show it programmatically by, let's say, clicking on a button, as the focus will be lost to the clicked button, the thumb label will disappear and appear again. Just realized that this is kind of normal (but I think it'd be better without this blinking)

• Closes Feat (Mat-Slider) : Focus method and blur Output #9332

[mmalerba]

The blinking is because you're blurring the slider when you click on the button and then programmatically re-focusing it. One thing that could potentially be done is to add a slight delay after blurring before hiding the thumb (let's not do it in this PR though)

[jelbourn]

@julianobrasil DM me your email address on Twitter?

[julianobrasil]

@jelbourn, I wasn't able to DM you (twitter policies), but my email address is public in github: https://github.com/julianobrasil

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