Use listener to update seekbar description async

Because multiple sources update the progress data, there can be race
events between the initial update and when the description updated. This
decouples those and uses a listener pattern to update the content
description separately from the progress state.

Fixes: 399825527
Flag: EXEMPT bugfix
Test: atest SeekBarViewModelTest
Test: manual - with and without scene container flag
Change-Id: I579bbcaadd825277d82dfb7fd4fd2cef3d870c1a