Updated the encoder flags for externally setting reference frames using
and updating to include latest changes in AV1.

1. For what reference frames to use, always initilize
cpi->ref_frame_flags with AOM_REFFRAME_ALL at the beginning of encoding
a frame. The internal ref_frame_flags starts from external flags. Added
AOM_EFLAG_NO_REF_LAST2 and AOM_EFLAG_NO_REF_LAST3 for LAST2 and LAST3.

2. For what reference frames to update, added ext_refresh_bwd_ref_frame
and ext_refresh_alt2_ref_frame for BWD and ALT2. Also, removed
AOM_EFLAG_FORCE_GF and AOM_EFLAG_FORCE_ARF since these are never
actually used. They can be added back if needed later.

Change-Id: I1e4429290f09bfcd1b26f2babc0cf556fc6fbc6c