Fix aom_write_bit() to match aom_read_bit().

The aom_write_bit() was not calling buf_uabs_write_bit() while the
 aom_read_bit() function was calling uabs_read_bit().

Change-Id: If98975341472988e8d809aa80a647d7a2531e21e
diff --git a/aom_dsp/bitwriter.h b/aom_dsp/bitwriter.h
index e1e5cc7..1c2e96b 100644
--- a/aom_dsp/bitwriter.h
+++ b/aom_dsp/bitwriter.h
@@ -112,7 +112,9 @@
 }
 
 static INLINE void aom_write_bit(aom_writer *w, int bit) {
-#if CONFIG_DAALA_EC
+#if CONFIG_ANS
+  buf_uabs_write_bit(w, bit);
+#elif CONFIG_DAALA_EC
   aom_daala_write_bit(w, bit);
 #else
   aom_write(w, bit, 128);  // aom_prob_half