MediaRouter: Use a lighter color for the group panel if needed

Added a logic to check the contrast between the text and its background
in the group volume panel. In case the contrast is insufficient (i.e. <
4.5), generate a 15% lighter color based on the app's primary color and
use it instead of the app's dark primary color.

Bug: 24336474
Change-Id: I6153335eee5b99400b8694a636e13b389e4fb7ea
3 files changed