Update WebAuthenticationBroker to make use of CustomTabs (Android) and ASWebAuthenticationSession (iOS)

[nickrandolph]

What would you like to be added:

Android implementation of WAB should use CustomTabs iOS implementation should use ASWebAuthenticationSession and fall back to SFAuthenticationSession

Why is this needed:

WAB should make use of latest guidance for Android and iOS

For which Platform:

• [ X ] iOS
• [ X ] Android
• [ ] WebAssembly
• [ ] WebAssembly renderers for Xamarin.Forms
• [ ] macOS
• [ ] Skia
  • [ ] WPF
  • [ ] GTK (Linux)
  • [ ] Tizen
• [ ] Windows
• [ ] Build tasks
• [ ] Solution Templates

Anything else we need to know?