Change "vp9" to "av1"

.gitignore

aom/aom.h

aom/aom_encoder.h

aom/aomcx.h

aom_dsp/sad.c

aom_dsp/x86/fwd_dct32x32_impl_avx2.h

aom_dsp/x86/fwd_dct32x32_impl_sse2.h

aomdec.c

aomenc.c

av1/av1_cx_iface.c

av1/av1_dx_iface.c

build/make/Makefile

examples/aom_temporal_svc_encoder.c

test/aom_temporal_svc_encoder.sh

test/aomdec.sh

test/aomenc.sh

test/decode_perf_test.cc

test/decode_to_md5.sh

test/decode_with_drops.sh

test/denoiser_sse2_test.cc