Add switch preference for Audio Description.

Preview: https://screenshot.googleplex.com/AXMFpGPPGPPBzXG

Bug: 214634651i
Test: Local flash and test
Test: atest SettingsRoboTests:AudioDescriptionPreferenceControllerTest

Change-Id: Ice2e2bc8ec4e77aba0cd486f854254009b91e541
6 files changed