Added missing file to Android.mk
Added missing file to Android.mk

I forgot to add a reference to backend.cpp in the Android makefile which
causes the build to fail.

Change-Id: Ibbeb29931c350d050c891ffbb557fc768f2f5c38

google/pindrop,eriser/pindrop,google/pindrop,eriser/pindrop,google/pindrop