Allow background color theming of toolbar menu

[sblatz]

It'd be great if we could theme this background color to better match the theme of Fenix!

┆Issue is synchronized with this Jira Task

[pocmo]

@sblatz Do you prefer to set the color in the theme or do you need something that is more dynamic?

[sblatz]

@pocmo Setting from the theme is fine as long as I know the attribute it's pulling from 😄

[pocmo]

We definitely need to add something custom. Under the hood we are using CardView and the menu inherits the CardView style (but we should make it possible to pick an independent background color):
https://android.googlesource.com/platform/frameworks/support/+/androidx-master-dev/cardview/res/values/styles.xml