vp9-svc: Fix for scene detection for SVC

vp9-svc: Fix for scene detection for SVC

For spatial layers: use the correct mi_cols/rows in the
scene detection. The scene detection for spatial layers
is only called once per superframe, but we were using wrong
mi_cols/rows (those for base spatial were being used).

Also increase frame_since_key threshold to account for spatial
layers.

Change-Id: I2731da49684a798c4718693a0468eda7db82d2bd