Fix: generate_gradle with vpython

Use vpython for `generate_gradle.py` and `list_java_targets.py`

Change-Id: I39bb601add8cc1c019a3aab9bca60bee0e341084
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315400
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791917}

build/android/gradle/generate_gradle.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.

build/android/list_java_targets.py

-#!/usr/bin/env python3
+#!/usr/bin/env vpython3
 # Copyright 2020 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.