Make exit signs stylable

[devotaaabel]

This relates to #972 . There are issues with the background color rendering on pre-API 21 devices.

[Guardiola31337]

@abhishek1508 @JunDai is this supported in the 1.0 UI SDK?

[abhishek1508]

Let's verify.
cc @JunDai @cafesilencio @langsmith

[abhishek1508]

After internal discussion with @asinghal22 we will not be prioritizing this work for v1.0. I am going to close this for now.
cc @Guardiola31337