Introducing API to produce slider events.

The API defines the production of slider events as a state flow. A
concrete implementation is also added in the form of a
SeekableSliderEventProducer, whose purpose is to produce a stream of
events from an Android SeekBar.

Test: atest SeekableSliderEventProducerTest
Bug: 295932558
Change-Id: Iae5d63f6bbc83d98ad12b3a115d3e8c712b82fdd