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
diff --git a/av1/decoder/decodemv.c b/av1/decoder/decodemv.c
index d6feaa8..8260f9d 100644
--- a/av1/decoder/decodemv.c
+++ b/av1/decoder/decodemv.c
@@ -746,7 +746,7 @@
 
   // 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;