FR: Respect global tint color in default FUI view controllers

[willbattel]

Step 1: Are you in the right place?

Yes

Step 2: Describe your environment

• Objective C or Swift: Swift
• iOS version: >=11.1
• Firebase SDK version: 6.27.0
• FirebaseUI version: 8.4.2
• CocoaPods Version: 1.9.2

Step 3: Describe the problem:

It would be nice if the default view controllers provided by FUI respected the global tint color. The current workaround is to make a custom view controller to hook into FUI, but for such a small change it seems overkill.

[yuchenshi]

New internal issue created for this: b/160627150