[TabBarView] Add accessibility label (#7727)

This adds the `accessibilityLabel` to MDCTabBarView, this falls back to the titleLabel.text if the accessibility label isn't set. This has been tested on a device.

## Testing
Steps to reproduce
1. Update the [example](https://github.com/material-components/material-components-ios/blob/develop/components/Tabs/examples/MDCTabBarViewTypicalExampleViewController.m) to have a custom `accessibilityLabel` for one of the tabBarItems
```objc
item1.accessibilityLabel = @"Hello world";
```
2. Select the item with Voiceover on
3. Navigate to the other items with titles but no custom value.

components/Tabs/src/TabBarView/MDCTabBarView.m

@@ -23,6 +23,7 @@
 
 static NSString *const kImageKeyPath = @"image";
 static NSString *const kTitleKeyPath = @"title";
+static NSString *const kAccessibilityLabelKeyPath = @"accessibilityLabel";
 
 @interface MDCTabBarView ()
 
@@ -92,6 +93,7 @@ - (void)setItems:(NSArray<UITabBarItem *> *)items {
     MDCTabBarViewItemView *itemView = [[MDCTabBarViewItemView alloc] init];
     itemView.translatesAutoresizingMaskIntoConstraints = NO;
     itemView.titleLabel.text = item.title;
+    itemView.accessibilityLabel = item.accessibilityLabel;
     itemView.titleLabel.textColor = [self titleColorForState:UIControlStateNormal];
     itemView.iconImageView.image = item.image;
     [itemView setContentCompressionResistancePriority:UILayoutPriorityRequired
@@ -226,13 +228,20 @@ - (void)addObserversToTabBarItems {
            forKeyPath:kTitleKeyPath
               options:NSKeyValueObservingOptionNew
               context:kKVOContextMDCTabBarView];
+    [item addObserver:self
+           forKeyPath:kAccessibilityLabelKeyPath
+              options:NSKeyValueObservingOptionNew
+              context:kKVOContextMDCTabBarView];
   }
 }
 
 - (void)removeObserversFromTabBarItems {
   for (UITabBarItem *item in self.items) {
     [item removeObserver:self forKeyPath:kImageKeyPath context:kKVOContextMDCTabBarView];
     [item removeObserver:self forKeyPath:kTitleKeyPath context:kKVOContextMDCTabBarView];
+    [item removeObserver:self
+              forKeyPath:kAccessibilityLabelKeyPath
+                 context:kKVOContextMDCTabBarView];
   }
 }
 
@@ -259,6 +268,8 @@ - (void)observeValueForKeyPath:(NSString *)keyPath
       tabBarItemView.iconImageView.image = change[NSKeyValueChangeNewKey];
     } else if ([keyPath isEqualToString:kTitleKeyPath]) {
       tabBarItemView.titleLabel.text = change[NSKeyValueChangeNewKey];
+    } else if ([keyPath isEqualToString:kAccessibilityLabelKeyPath]) {
+      tabBarItemView.accessibilityLabel = change[NSKeyValueChangeNewKey];
     }
   } else {
     [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];

components/Tabs/src/TabBarView/private/MDCTabBarViewItemView.m

@@ -132,4 +132,10 @@ - (CGSize)sizeThatFits:(CGSize)size {
   return CGSizeMake(width, height);
 }
 
+#pragma mark - UIAccessibility
+
+- (NSString *)accessibilityLabel {
+  return [super accessibilityLabel] ?: self.titleLabel.text;
+}
+
 @end