[Android] Stop speaking current announcement before announcing something else.

TalkBack announces successive AccessibilityEvent.TYPE_ANNOUNCEMENT
events by adding rather than flushing the queue.

With Android’s migration away from using ChromeVox for WebViews, this
became more noticeable.

Adding AccessibilityManager.interrupt() before the announcement stops
any speech that may be in progress, before making an announcement.

src/android/com/phonegap/plugin/mobileaccessibility/JellyBeanMobileAccessibilityHelper.java

@@ -40,6 +40,7 @@ public class JellyBeanMobileAccessibilityHelper extends
     @Override
     public void announceForAccessibility(CharSequence text) {
         if (mAccessibilityManager.isEnabled() && mParent != null) {
+            mAccessibilityManager.interrupt();
             AccessibilityEvent event = AccessibilityEvent.obtain(
                     AccessibilityEvent.TYPE_ANNOUNCEMENT);
             mWebView.onInitializeAccessibilityEvent(event);