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)

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