Add android-app scheme to valid scheme list

[scottbullard]

Addresses referer values such as: android-app://com.google.android.gm

#131

[snowplowcla]

Thanks for your pull request. Is this your first contribution to a Snowplow open source project? Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[snowplowcla]

We will have to close this PR in 7 days unless the CLA can be signed.

[BenFradet]

@scottbullard did you sign the cla in the end?

[dsingleton]

I've run into the same issue, would you be open to someone submitting the same change, but signing the CLA?

[alexanderdean]

Hi @dsingleton - as long as you are duplicating the intent of this PR but not the literal code, it should be fine. Closing this PR.