Add content description to volume title in settings.

Adds content description that is announced by talkback when a11y focus
is on volume preference. This improves talkback announcement when view changes.

Fixes: 285529113
Bug: 285455826
Fixes: 285487766
Test: atest VolumeSeekBarPreferenceControllerTest
Test: atest VolumeSeekBarPreferenceTest
Change-Id: Ibe80b4b1d489dc058df1cc79c96b034d5ddc6e56