Code class0 using aom_read() / aom_write().

The av1_mv_class0_tree is a balanced tree with two leafs and can
 simply be coded as a boolean with probability class0[0].
If CLASS0_SIZE is ever changed from 1, this change will need to be
 reverted.

Change-Id: If294dac825a5f945371092c74aa8e3f84cd962b6

av1/decoder/decodemv.c

@@ -503,7 +503,7 @@ static int read_mv_component(aom_reader *r, const nmv_component *mvcomp,
 
   // Integer part
   if (class0) {
-    d = aom_read_tree(r, av1_mv_class0_tree, mvcomp->class0, ACCT_STR);
+    d = aom_read(r, mvcomp->class0[0], ACCT_STR);
     mag = 0;
   } else {
     int i;

av1/encoder/encodemv.c

@@ -23,12 +23,10 @@
 static struct av1_token mv_joint_encodings[MV_JOINTS];
 static struct av1_token mv_class_encodings[MV_CLASSES];
 static struct av1_token mv_fp_encodings[MV_FP_SIZE];
-static struct av1_token mv_class0_encodings[CLASS0_SIZE];
 
 void av1_entropy_mv_init(void) {
   av1_tokens_from_tree(mv_joint_encodings, av1_mv_joint_tree);
   av1_tokens_from_tree(mv_class_encodings, av1_mv_class_tree);
-  av1_tokens_from_tree(mv_class0_encodings, av1_mv_class0_tree);
   av1_tokens_from_tree(mv_fp_encodings, av1_mv_fp_tree);
 }
 
@@ -53,8 +51,7 @@ static void encode_mv_component(aom_writer *w, int comp,
 
   // Integer bits
   if (mv_class == MV_CLASS_0) {
-    av1_write_token(w, av1_mv_class0_tree, mvcomp->class0,
-                    &mv_class0_encodings[d]);
+    aom_write(w, d, mvcomp->class0[0]);
   } else {
     int i;
     const int n = mv_class + CLASS0_BITS - 1;  // number of bits