iOS: Improve accessibility of multiline TextInputs

Summary:
Fixes #13691.

Prior to this change, VoiceOver generally didn't say anything when selecting a multiline TextInput. The only exception was VoiceOver would announce the TextInput's placeholder if it had one set.

After this change, VoiceOver announces the following for multiline TextInputs:
  - The TextInput's content.
  - The TextInput's placeholder if the TextInput doesn't have any content in it.
  - The fact that the TextInput is a textfield.
  - The cursor position if the TextInput is being edited.

This is similar to the behavior of single line TextInputs.

This change achieves this by disabling `RCTTextView` as an accessibility element. `RCTTextView` is a subclass of `RCTView` so VoiceOver doesn't recognize this as a textfield. Instead, VoiceOver now sees the child `RCTUITextView` which is a subclass of `UITextView` so VoiceOver does recognize it as a textfield. Additionally, an `accessibilityLabel` implementation was added to `RCTUITextView` in order to take the value of the placeholder into account.

Verified the announcements of TextInputs with various props:
  - No placeholder and no content
  - Just a placeholder
  - Just content
  - Both a placeholder and content

Did this for both singe line inputs and multiline inputs. For setting content in multiline inputs, I tested both using the `value` prop and passing children. All other props being equal, these configurations resulted in similar announcements.

I verified that the following accessibility props work the same on singleline and multiline TextInputs:
  - `accessible`
  - `accessibilityLabel`
  - `accessibilityTraits`
  - `accessibilityViewIsModal`
  - `onAccessibilityTap`
  - `onMagicTap`

Additionally, my team has been using this change in our app.

Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/14200

Differential Revision: D5185727

Pulled By: shergin

fbshipit-source-id: 94271e6c8b089eb82006b52fe7917649d69e74af

Libraries/Text/RCTUITextView.m

@@ -52,6 +52,25 @@ static UIColor *defaultPlaceholderTextColor()
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }
 
+- (NSString *)accessibilityLabel
+{
+  NSMutableString *accessibilityLabel = [NSMutableString new];
+  
+  NSString *superAccessibilityLabel = [super accessibilityLabel];
+  if (superAccessibilityLabel.length > 0) {
+    [accessibilityLabel appendString:superAccessibilityLabel];
+  }
+  
+  if (self.placeholderText.length > 0 && self.text.length == 0) {
+    if (accessibilityLabel.length > 0) {
+      [accessibilityLabel appendString:@" "];
+    }
+    [accessibilityLabel appendString:self.placeholderText];
+  }
+  
+  return accessibilityLabel;
+}
+
 #pragma mark - Properties
 
 - (void)setPlaceholderText:(NSString *)placeholderText