Move pick_frame_size() to ratectrl.c

This is a first step in cleaning up the redundancies between
vp8_calc_{auto_,}iframe_target_size. The pick_frame_size() function is
moved to ratectrl.c, and made to be the primary interface. This means
that the various calc_*_target_size functions can be made private.

Change-Id: I66a9a62a5f9c23c818015e03f92f3757bf3bb5c8