Set activity indicator color to white (#1212)

Resolves #1207 caused by changes in #1194

FirebaseAuthUI/Sources/FUIAuthBaseViewController.m

@@ -132,6 +132,7 @@ + (UIActivityIndicatorView *)addActivityIndicator:(UIView *)view {
   UIActivityIndicatorView *activityIndicator =
       [[UIActivityIndicatorView alloc]
        initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleLarge];
+  activityIndicator.color = [UIColor colorWithWhite:1 alpha:1];
   UIView *tintView = [[UIView alloc] initWithFrame:CGRectInset(activityIndicator.frame,
                                                                -kActivityIndiactorPadding,
                                                                -kActivityIndiactorPadding)];