handle_inter_mode: add handle_newmv()
handle_inter_mode: add handle_newmv()

Refactors newmv handling code in handle_inter_mode into its own
function. This makes handle_inter_mode shorter and easier to
understand.

Change-Id: I152a61ee8a7f323936b4a772f14f8e286724443d

GrokImageCompression/aom,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom