acloud: Handle branch & build_target in AVDSpec if user doesn't specify branch or build_target.

Bug: 112616295
Test: ./run_tests.sh, m acloud && acloud create
acloud create --branch master

Change-Id: I87d48687f8628b65935b3e50f74eaf9eb375844d
diff --git a/internal/constants.py b/internal/constants.py
index e8185c5..7d1820b 100755
--- a/internal/constants.py
+++ b/internal/constants.py
@@ -37,6 +37,13 @@
 IMAGE_SRC_REMOTE = "remote_image"
 IMAGE_SRC_LOCAL = "local_image"
 
+# AVD types in build target
+AVD_TYPES_MAPPING = {
+    TYPE_GCE: "gce",
+    TYPE_CF: "cf",
+    TYPE_GF: "sdk",
+}
+
 # Instance types
 INSTANCE_TYPE_REMOTE = "remote"
 INSTANCE_TYPE_LOCAL = "local"