Fix problem with BootAnimation exit

After the completion of each animation part, the boot animation must
sleep for the number of frames specified in part.pause. If this is
done in a single sleep statement, it is possible that the boot animation
will continue to display long after surface flinger has finished booting.

To address that, the pause between parts is done in a loop and on each
iteration, the property that surface flinger sets to indicate that animation
should exit is checked.  This minimizes, but doesn't remove, the likelihood
that the bootanimation is visible when the UI comes up.

Change-Id: Ib71df0a72c0a47a1f7231b02379dc951c1f93e52
Signed-off-by: Russell Webb <russell.webb@intel.com>