vp9: Reset rc flags on some configuration changes.

vp9: Reset rc flags on some configuration changes.

For large dynamic changes in target avg_frame_bandwidth, or
a change in resolution, via the update in change_config()),
reset the under/overshoot flags (rc_1_frame, rc_2_frame)
to prevent constraining the QP for the first few frames
following the change.

For SVC use the spatial stream avg_frame_bandwidth in
reset condition.

For the avg_frame_bandwidth condition, use fairly large
threshold (~50%) for now in reset.

This allows for better/faster QP response if, for example,
application dynamically changes bitrate by large amount.

Change-Id: Ib6e3761732d956949d79c9247e50dba744a535c0