Modified spatial scalable encoder & unit tests

Modifications to the spatial scalable encoder to match
changes made to the scaling code in the decoder.

In particular, the use of a dummy first frame was removed
now that the decoder is able to handle a smaller first
frame.

SvcTest.FirstFrameHasLayers unit test re-enabled.

Change-Id: Ic2e91fbe4eadf95895569947670d36d68abaf458