explicitly set max heap size when running signapk

On the mac, -Xmx defaults to 64MB, which isn't enough to do a
whole-file signing of a full OTA package.

tools/releasetools/common.py

@@ -208,7 +208,7 @@ def SignFile(input_name, output_name, key, password, align=None,
   else:
     sign_name = output_name
 
-  cmd = ["java", "-jar",
+  cmd = ["java", "-Xmx512m", "-jar",
            os.path.join(OPTIONS.search_path, "framework", "signapk.jar")]
   if whole_file:
     cmd.append("-w")