[ios] Allow MGLScaleBar to support dark mode on iOS 13

[captainbarbosa]

Fixes #15452 by changing the text color before being drawn into the CGContext.

[friedbunny]

Can you explain what the underlying issue was? Why is self.textColor now contingent on dark mode?

[captainbarbosa]

Can you explain what the underlying issue was?

Here's what I think happened - the color of aUITextLabel can't adjust whether or not a device is in dark mode when using a fixed color like [UIColor whiteColor]. So, when switching to dark mode the color remains white, and it then gets drawn into the CGContext, further cementing its permanent color.

Since the inner text color should remain black no matter if the device is in dark mode or not, we don't need to determine if the device is in dark mode at all (as you mentioned in #15524 (review)), since we'll always use a fixed color for now.

In the future, if we do decide to support dark more here more dynamically, perhaps one option is to use [UIColor systemWhite], which appears to handle responses to the current trait environment (and thus dark mode).