vp9-svc: Add layer bitrate targeting to SVC datarate tests.

vp9-svc: Add layer bitrate targeting to SVC datarate tests.

Modify and update the SVC datarate unittests to verify the
rate targeting for each spatial-temporal layer.
The current tests were only verifying the rate targeting
of the full SVC stream, not individual layers.
Also re-enabled a test that was disabled.

This is a stronger verification of the layered rate control
for SVC for 1 pass CBR encoding.

Added PostEncodeFrameHook, needed to get the layer_id and
update the layer buffer level.

Change-Id: I9fd54ad474686b20a6de3250d587e2cec194a56f