Prevent VABC from being disabled on release-key builds

VABC is mandated now, and plain VAB will be unsupported later.

Test: th
Change-Id: Iad19433c06b25246d4f7bffe5599035836e36076

tools/releasetools/common.py

@@ -449,6 +449,11 @@ class BuildInfo(object):
     system_prop = self.info_dict.get("system.build.prop")
     return system_prop and system_prop.GetProp("ro.build.version.release") == "11"
 
+  @property
+  def is_release_key(self):
+    system_prop = self.info_dict.get("build.prop")
+    return system_prop and system_prop.GetProp("ro.build.tags") == "release-key"
+
   @property
   def vabc_compression_param(self):
     return self.get("virtual_ab_compression_method", "")