Fix display list issue that was crashing an external app.

The app was removing a View whilst in its onDraw() method. This meant
that we asked it for its display list and it invalidated that display list
(by removing itself) before it returned from onDraw(). We later attempted to
draw that invalid display list into its parent nad died in native code.

The fix is to check the state of the display list after the call to getDisplayList()
and to avoid doing further work with it if it's invalid.

Change-Id: I14a342b4fe79c8dce2626ff61237b447040e7f42