Proper type for RCTBackedTextInputViewProtocol::defaultTextAttributes

Summary:
This diff changes how we apply default text attributes to backed text input.
The original change in https://github.com/facebook/react-native/pull/23585 that introduced the `reactTextAttributes` field in for RCTBackedTextInputViewProtocol was great! Thank you Wu zhongwuzw !
However, there is one detail that needs to be changed.
RCTBackedTextInputViewProtocol is designed to only abstract complexity of iOS text input components (UITextView and UITextField); it intentionally does not have any React-specific fields or types. Adding a field `RCTTextAttributes *reactTextAttributes;` violates this principle and make it hard to reuse this functionality in the new Fabric-powered TextInput.

This diff changes the type of this prop from `RCTTextAttributes` to `NSDictionary<NSAttributedStringKey,id> *`  (exact same type that UITextView and UITextField use).

Reviewed By: cpojer

Differential Revision: D17408501

fbshipit-source-id: 65f2bba119ccc30f22e87c28d0f8ea6f731cd365

Libraries/Text/TextInput/RCTBaseTextInputShadowView.m

@@ -47,6 +47,16 @@
   return YES;
 }
 
+- (void)didSetProps:(NSArray<NSString *> *)changedProps
+{
+  [super didSetProps:changedProps];
+
+  // `backgroundColor` and `opacity` are being applied directly to a UIView,
+  // therefore we need to exclude them from base `textAttributes`.
+  self.textAttributes.backgroundColor = nil;
+  self.textAttributes.opacity = NAN;
+}
+
 - (void)layoutSubviewsWithContext:(RCTLayoutContext)layoutContext
 {
   // Do nothing.