Refactor of the SeekableSliderEventProducer and SeekableSliderTracker.

To support more generalized slider haptics, the
SeekableSliderEventProducer has been refactored to a
SliderStateProducer. The SeekableSliderTracker has also been renamed to
better convey its functionality. The only element that remains coupled
to an Android SeekBar is the SeekableSliderHapticPlugin.

Test: atest SystemUITests
Flag: NONE
Bug: TBD

Change-Id: I62e122fe44582ad4af1608924b09c23ad1a25453