Add parameter to allow user to set bottom bar backgrond color along w…

[krishnajithapa]

There was an issue reported where user was not able to see bottom bar button when background color was white. So i have added parameter to let user specify the background and button color.

[diegotori]

@krishnajithapa thanks for your PR.

I think that the changes should be present in ChewieController if they are not overridden in Chewie by the user.

In other words, ChewieController should hold immutable Color properties related to this PR, and they should be used in MaterialControls as well as MaterialDesktopControls. If those two classes specify an optional custom color value, then use those values, otherwise fallback to the controller.

Chewie, PlayerWithControls, and AdaptiveControls should funnel the user-defined optional Color properties into MaterialControls and MaterialDesktopControls.