fix(slider): not reacting to directionality changes

[crisbeto]

Fixes the slider not updating if its direction is changed dynamically. This is something I came across while testing #6641.

[mmalerba]

I believe @g1shin is working on this in her PR for #6542

[crisbeto]

#6542 seems to be related to the focus styles, not the directionality, although the fix should be somewhat similar (adding in a missing markForCheck somewhere).

[mmalerba]

yeah she's just fixing the various OnPush issues: #6700

[crisbeto]

Alright, closing in that case.

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