This commit separate the frame index of EXT_ARFs' from other frame
types in the ext-refs setting.

It improves the average RD performance by

0.206% in the lowres, and
0.173% in the midres.

The overall gains for the ext-refs compared to the baseline are

5.665% in the lowres, and
4.883% in the midres.

Change-Id: I6591ad29120880c1aef0bd0b7cf15238c3f3b8f3