[Slider] Add support for secondary track

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**Is your feature request related to a problem? Please describe.**

I want to be able to show the cache buffer in a slider that allows to control a player. The value is informational and should not limit the ability to slide to values larger than the buffer.

**Describe the solution you'd like**
Behave like SeekBar and propose a secondary track to represent that value to users.

**Describe alternatives you've considered**
As all is private there's no way to implement that by extending the slider, alternative are seekbar with custom labels.

Start by locating the Slider and SeekBar implementations and compare how their tracks and values are represented. Add support for an informational secondary track that shows the buffer value without preventing the slider from moving beyond it, then verify the behavior against the existing component tests if available.

Written by the indexing model from the issue text.