Add supports for translucent nav bar

This should be enough to fix the top padding found in the auth UI when setting the nav bar non translucent.

Tested on multiple devices (with and without notch).

These properties require iOS 7+

Author: SebastienFCT

Auth/FirebaseAuthUI/FUIAuthPickerViewController.m

@@ -91,6 +91,11 @@ - (instancetype)initWithNibName:(NSString *)nibNameOrNil
 - (void)viewDidLoad {
   [super viewDidLoad];
 
+  // Makes sure that embedded scroll view properly handles translucent navigation bar
+  if (!self.navigationController.navigationBar.isTranslucent) {
+    self.extendedLayoutIncludesOpaqueBars = true;
+  }
+
   if (!self.authUI.shouldHideCancelButton) {
     UIBarButtonItem *cancelBarButton =
         [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemCancel