gn2bp: add update value arg support

Allows for passing in a function that will be applied to transform the
arg value.

Test: none
Change-Id: Iff5c5ab62f8f93d15ff0fb36d740bb326b08655e

tools/gn2bp/gen_android_bp

@@ -656,6 +656,9 @@ class BaseActionSanitizer():
     i = self.target.args.index(arg)
     self.target.args[i + 1] = value
 
+  def _update_value_arg(self, arg, func):
+    self._set_value_arg(arg, func(self._get_value_arg(arg)))
+
   def _set_arg_at(self, position, value):
     self.target.args[position] = value