MediaRouter: New color scheme for main and group media controls

Instead of showing dark controls in a possibly dark (i.e. the primary
dark), modeled the white dialog and used the the primary color for the
group controls. Also, reverted opacity values back to 87% for better
contrast in a light background. In addition,
- Fixed a bug that the volume slider for a disabled media route is still
  movable.
- Fixed a bug that the volume sliders are always tinted with the primary
  color.
- Fixed a bug that the ripple drawables for the play/pause and group
  collapse/expand buttons are cut off by an invisible divider.
- Corrected the inconsistent disabled alpha values.
- Ensured the volume slider color is opaque so that the underlying
  progress bar is not seen through the thumb.

Bug: 24336474, Bug: 24779916, Bug: 24779734, Bug: 24779945
Change-Id: Ic163d73dd38558994ff5e0bf8b4c3d4b66800c23
8 files changed