codegen: execute using bash

On Ubuntu 18.04, /bin/sh is symlinked to dash. Executing codegen under
dash results in the following error:

  codegen: Syntax error: "(" unexpected

To prevent this, explicitly use bash.

Test: manual ($ANDROID_BUILD_TOP/development/scripts/codegen --help)
Change-Id: I11bd5e41757a8d26f5e9d7d3340306d5ff4da1f9
diff --git a/scripts/codegen b/scripts/codegen
index 799347c..d53dd37 100755
--- a/scripts/codegen
+++ b/scripts/codegen
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 top="$(dirname $0)/../.."