RN: Announce "unselected" Accessibility State (Android)

Summary:
Changes React Native so that when `accessibilityState` is used to change a view from `selected: true` to `selected: false`, the change in state is announced.

This is how `checked` works; it is unclear why Android does not do this for `selected`, too.

Changelog:
[Android][Added] - TalkBack now announces "unselected" when changing `accessibilityState.selected` to false.

Reviewed By: blavalla

Differential Revision: D27449293

fbshipit-source-id: a6d77b55d63655973ad93c4d5e3743742501f378

Author: yungsters

ReactAndroid/src/main/java/com/facebook/react/uimanager/BaseViewManager.java

@@ -167,7 +167,17 @@ public void setViewState(@NonNull T view, @Nullable ReadableMap accessibilitySta
       return;
     }
     if (accessibilityState.hasKey("selected")) {
-      view.setSelected(accessibilityState.getBoolean("selected"));
+      boolean prevSelected = view.isSelected();
+      boolean nextSelected = accessibilityState.getBoolean("selected");
+      view.setSelected(nextSelected);
+
+      // For some reason, Android does not announce "unselected" when state changes.
+      // This is inconsistent with other platforms, but also with the "checked" state.
+      // So manually announce this.
+      if (view.isAccessibilityFocused() && prevSelected && !nextSelected) {
+        view.announceForAccessibility(
+            view.getContext().getString(R.string.state_unselected_description));
+      }
     } else {
       view.setSelected(false);
     }

ReactAndroid/src/main/res/views/uimanager/values/strings_unlocalized.xml

@@ -92,6 +92,10 @@
     name="state_collapsed_description"
     translatable="false"
     >collapsed</string>
+  <string
+    name="state_unselected_description"
+    translatable="false"
+    >unselected</string>
   <string
     name="state_on_description"
     translatable="false"