Make Accessibility Label Nullable

Summary:
After chatting with Brett Lavalla on D14791043 we determined that on Android it makes sense to have a text span with an accessibility role but *without* an accessibility description.

This diff changes the `AccessibleClickSpan` class to reflect this. In this diff I change all `accessibilityDescription` fields to have a `Nullable` annotation with the exception of the `setAccessibilityDescription` method (I think it's sensible to avoid having the function become a no-op).

Reviewed By: usikder

Differential Revision: D14792206

fbshipit-source-id: 7d2da9c5b83fac54e078b6ec3e234ad2750fc012

litho-widget/src/main/java/com/facebook/litho/widget/accessibility/AccessibleClickableSpan.java

@@ -28,10 +28,10 @@
  * want to set the accessibility label to the corresponding plurals resource.
  */
 public abstract class AccessibleClickableSpan extends ClickableSpan {
-  private String mAccessibilityDescription;
+  private @Nullable String mAccessibilityDescription;
   @AccessibilityRole.AccessibilityRoleType @Nullable private String mAccessibilityRole;
 
-  public String getAccessibilityDescription() {
+  public @Nullable String getAccessibilityDescription() {
     return mAccessibilityDescription;
   }
 
@@ -49,12 +49,12 @@ public void setAccessibilityRole(
     mAccessibilityRole = accessibilityRole;
   }
 
-  public AccessibleClickableSpan(String accessibilityDescription) {
+  public AccessibleClickableSpan(@Nullable String accessibilityDescription) {
     this(accessibilityDescription, null);
   }
 
   public AccessibleClickableSpan(
-      String accessibilityDescription,
+      @Nullable String accessibilityDescription,
       @Nullable @AccessibilityRole.AccessibilityRoleType String accessibilityRole) {
     super();
     mAccessibilityDescription = accessibilityDescription;