vp9-svc: Improve frame dropper for spatial layers.

vp9-svc: Improve frame dropper for spatial layers.

SVC frame dropper: modify the logic to allow for individual
spatial layers to drop. This removes the constraint that all
upper spatial layers must drop when a given spatial layer drops.

Add a flag to the pkt to indicate whether a spatial layer is
encoded or dropped. This is needed for applications that enable
this feature (frame dropping for SVC).

For a current spatial layer, if its previous spatial layer is
dropped, then disable certain features for that layer:
inter-layer prediction, base_mv, partition_reuse, copy partition.

Also add the constraint to never drop a spatial layer if its
base layer is a key frame.

Updates to sample encoder (vp9_spatial_svc_encoder) and the
SVC datarate unittests to properly handle frame dropping.

Bump up ABI version.
Change-Id: I7d14ccf67b8d014a7abfce5ba3989fc623e94067