gn2bp: add _append_arg helper

Decided to name it _append_arg as this works for value args and list
args, and we could technically overload it to support flags if needed.

Test: none
Change-Id: If44ad2ebb01bb3db86382dc02d5543838a70ede9

tools/gn2bp/gen_android_bp

@@ -670,6 +670,10 @@ class BaseActionSanitizer():
     if hasValue:
       self.target.args.pop(i)
 
+  def _append_arg(self, arg, value):
+    self.target.args.append(arg)
+    self.target.args.append(value)
+
   # wrap filename in location tag.
   def _location_tag(self, filename):
     return '$(location %s)' % filename