vp9-svc: Fix issue with reseting lst_fb_idx.

vp9-svc: Fix issue with reseting lst_fb_idx.

When encoding a given spatial layer and the same spatial layer
on previous superframe was dropped (or disabled due to 0 bitrate),
the lst_fb_idx for current layer is set to the buffer index that
was last updated on TL0 frame (for the same spatial layer).

This condition was to maintain proper temporal prediction pattern
under frame drops, and it should only apply to INTER frames.

But the condition was causing an assert to be triggered on spatial
layers whose base are key frames. Fix is to condition this reset of
lst_fb_idx on the "is_key_frame" flag. Also initialize the
fb_idx_upd_tl0 to -1 and only use it for a given spatial layer
if its been set.

These issues can happen when superframe drop happens just before
a key frame, or when stream starts with lower layers and dynamically
enabled higher spatial layers.

Added datarate unittest the inserts key frame after superframe drop,
and verified that this fix is needed for test to pass.
Also modified the existing DisableEnable spatial layer test to trigger
the issue of using fb_idx_upd_tl0 when it hasn't been set for a
spatial layer.

Change-Id: I059d1135736aca17e1326b9b4a2b16371eb4634e