Expose keyboard height as view inset on iOS (flutter#4428)

Keyboard height is now treated as a view inset, which could be used to
shrink the content area of the app as is done in the Material Scaffold,
rather than padding, which is used to indicate content areas where user
interaction should be avoided.

shell/platform/darwin/ios/framework/Source/FlutterViewController.mm

@@ -641,9 +641,7 @@ - (void)updateViewportPadding {
     _viewportMetrics.physical_padding_top = self.view.safeAreaInsets.top * scale;
     _viewportMetrics.physical_padding_left = self.view.safeAreaInsets.left * scale;
     _viewportMetrics.physical_padding_right = self.view.safeAreaInsets.right * scale;
-    // TODO(cbracken): Once framework has been updated to use view insets for the keyboard, enable
-    // bottom safe area padding.
-    // _viewportMetrics.physical_padding_bottom = self.view.safeAreaInsets.bottom * scale;
+    _viewportMetrics.physical_padding_bottom = self.view.safeAreaInsets.bottom * scale;
 #pragma clang diagnostic pop
   } else {
     _viewportMetrics.physical_padding_top = [self statusBarPadding] * scale;
@@ -657,19 +655,11 @@ - (void)keyboardWillChangeFrame:(NSNotification*)notification {
   CGFloat bottom = CGRectGetHeight([[info objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue]);
   CGFloat scale = [UIScreen mainScreen].scale;
   _viewportMetrics.physical_view_inset_bottom = bottom * scale;
-
-  // TODO(cbracken): Once framework has been updated to use view insets for keyboard padding,
-  // eliminate this line.
-  _viewportMetrics.physical_padding_bottom = bottom * scale;
   [self updateViewportMetrics];
 }
 
 - (void)keyboardWillBeHidden:(NSNotification*)notification {
   _viewportMetrics.physical_view_inset_bottom = 0;
-
-  // TODO(cbracken): Once framework has been updated to use view insets for keyboard padding,
-  // eliminate this line.
-  _viewportMetrics.physical_padding_bottom = 0;
   [self updateViewportMetrics];
 }