This commit makes the encoder to use different frame context index
for different frame types. In the baseline setting, it sets the
frame context index of the overlay frame to be different from other
regular inter frames. In the ext-refs setting, it further allows
the backward reference frame to use a different index.

It improves the compression performance for both settings.

Baseline
lowres  0.12%

ext-refs
lowres  0.50%
midres  0.56%

Change-Id: I7c63ddec9fc296c56a86353cf2c661a740b97a97