When temporal layers is used (i.e., number_of_layers > 1),
we don't use the frame rate boost for setting the key
frame target size. The factor was forcing the target size to be
always at its minimum (2* per_frame_bandwidth) for low frame rates
(i.e., base layer frame rate).

Generally we should modify or remove this frame rate factor;
for now we turn if off for number_of_layers > 1.

Change-Id: Ia5acf406c9b2f634d30ac2473adc7b9bf2e7e6c6