MediaRouter: Use volume slider thumb provided by AppCompatSeekBar

Appcompat uses its own thumb only for pre-API 21 (Lollipop, before
material) which doesn't have touch animation support while material has
it. Follow AppCompatSeekBar to keep consistency.

After this CL, disabled volume sliders may look differently per API
levels.

Bug: 24212201, Bug: 24553329
Change-Id: I1eca61baf735246f38db5f5b1bff5354c88ff407
3 files changed