Adds an adaptive constructor for the Material Slider. An adaptive widget is one that renders itself as Material on Android, and Cupertino on iOS. This work is based off of a similar feature on Switches: bbb080b3#diff-fe2bb980c6207699cbf45538fe927afa.

The motivation for this change is that we should provide adaptive constructors for as many widgets as necessary in the Material library. In Material, it is suggested that the slider is an iOS-style slider.