Provide synchronization to setview to avoid NPE

If setView() will be called from two different threads
then mView property of a View object may have inconsistent
value. For instance, setView() may set mView to null causing
NullPointerException. Synchronize root.setView() as well to
avoid this.

Change-Id: I5f9cf47ece5d4aca575bd8644ecfcee0ed43d843