Always update mute icon along with slider value.

Previously, if the media device volume was updated during dragging
the slider thumb, the mute icon might become out-of-sync with the slider
position.

Fix: 417319884
Flag: EXEMPT trivial UI bugfix
Test: atest MediaOutputAdapterTest, Verified on a device.
Change-Id: I819e4904f415246fba3d8d22a97e48b3e7111627