Update seekbar content description and process in background

Updates the content description of the seekbar times to match the Chronometer
implementation, and moves the formatting to the background

Fixes: 392039095
Test: atest SeekBarViewModelTest
Test: atest SeekBarObserverTest
Test: verify with TalkBack
Flag: EXEMPT bugfix
Change-Id: Ie441f1810e0a1709427038ef5c99f8ab6bfc026f