Show volume percentage only for touch interaction.

Fixes the scenario when a user changes volume with a Talkback swipe gesture which results in calling `onProgressChanged(fromUser = true)`, however, no other slider events like `onStopTrackingTouch` are invoked.

The video of the fixed UI: http://b/374857557#attachment61633177.

Bug: 374857557
Flag: EXEMPT bugfix
Test: atest MediaOutputAdapterTest
Change-Id: I1960d91b19f4f033419be95bf5bcb74625cc0be5