fix(FirebaseApp): Return app if it exists

[ValentinFunk]

Checklist

• Issue number for this PR: Problems with hmr #518, Angularfire2 SDK3 and Hot Module Reloading #235
• Docs included?: no
• Test units included?: yes
• e2e tests included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully?

Description

This supports HMR by returning the existing app instance if it was already initialized. (See #518, #235)

Fixes errors such as

No App '[DEFAULT]' has been created - call App.initializeApp() (app/no-app).

[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).

📝 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.

[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).

📝 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.

[ValentinFunk]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[jamesdaniels]

LGTM