Fix slider volume rendering for group playback.

The bug is caused by broken slider volume animation. This change
removes the code that tries to start the animation.

Flag: EXEMPT bugfix
Bug: 399845811, 399603559
Fix: 399603559, 399603559
Test: atest SystemUITests:com.android.systemui.media.dialog,
atest SystemUIGoogleScreenshotTests:com.android.systemui.media.dialog

Change-Id: I00c51157dcfad0285ae3bd8946fa9ef423d32457