made ButtonClicker target abi armeabi-v7a rather than arm64-v8a

[jknight-nc]

https://github.com/playgameservices/cpp-android-basic-samples/issues/32

I don't think ButtonClicker should target arm64-v8a by default. That ABI is very niche. Most devices can emulate armeabi-v7a if they dont support it natively. This makes much more sense.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

:memo: Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[jknight-nc]

I'm going to sign this from my personal email and github account.