Adds platform-specific IDs, new "show interstitial" button to AdMob example

[redbrogdon]

This is a clean redo of PR #312, of which I made a mess when attempting to rebase onto upstream changes. There shouldn't be anything in here other than code changes to the firebase-admob example, a version bump to the pubspec, and a corresponding CHANGLEOG update.

The changes to the example are:

• Load and Show functionality for interstitials has been separated into two separate buttons, which is more in line with our best practices for AdMob.
• Separate, platform-specific ad unit IDs and App IDs have been added.

[HansMuller]

These changes look good, thank you!

Once Travis approves I'll update the version number, land the PR, and publish a new version of the plugin.

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[redbrogdon]

Thanks!

[HansMuller]

Published as 0.2.2 https://pub.dartlang.org/packages/firebase_admob

[HansMuller]

I will review #328 soon, a little swamped at the moment

[redbrogdon]

No problem. I'm mostly done with the iOS and Android implementations, and was planning to add them to the PR in a day or two, so it'd probably be more efficient to hold off anyway.