fix sign_target_files_apks on non-user builds

Not that these should ever be signed, but still, it shouldn't crash.

Change-Id: I2b837e2c77f6e81743f93c96f361fc187b4fb82b

tools/releasetools/sign_target_files_apks

@@ -237,6 +237,8 @@ def RewriteProps(data):
         value = value.split()
         if len(value) == 2 and value[1].endswith("-keys"):
           value = value[0]
+        else:
+          value = " ".join(value)
       line = key + "=" + value
     if line != original_line:
       print "  replace: ", original_line