Add synchronization around the listeners in ShadowClipboardManager without synchronization tests fail:-

java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:859)
	at java.util.ArrayList$Itr.next(ArrayList.java:831)
	at org.robolectric.shadows.ShadowClipboardManager.setPrimaryClip(ShadowClipboardManager.java:27)
	at android.content.ClipboardManager.setPrimaryClip(ClipboardManager.java)
Address review comments - prefer CopyOnWriteArrayList over our own synchronization
Merge pull request #1120 from jongerrish/synchronize_shadow_clipboard_manager

Add synchronization around the listeners in ShadowClipboardManager