Add cross platform IAP status sync and tracking.

[khaykov]

This PR adds some tracking and a subscription status sync using Symperium. We are using existing Preferences bucket, to which we added subscription related fields.

Simperium has a totaly different mindset when it comes to networking, so I'm not sure I utilized it in the best possible way, but it works so far.

Before testing:

• Make sure you are logged in to the latest version of Play Store.
• Make sure you are using release version of the app (or the debug version without .debug preffix).
• Let me know what account you will be using for purchase testing so I can add it in Play Store. This will allow you to use test cards.
• Subscriptions renew every five minutes. You can cancel them from Subscription screen of Play Store.

Since we are using Simperium bucket to determine visibility of IAP banner the testing steps are mostly the same:

• Test purchase flow from sidebar with working test card. Confirm that after purchase is successful, "Thank you!" snackbar appears, Sustainer banner is not visible in sidebar, and black "Thank you" banner is visible in settings.
• Test purchase flow from Settings screen with working test card. Confirm that after purchase is successful, "Thank you!" snackbar appears, Sustainer banner is replaced with "Thank you" banner in Settings screen and it is not visible in Sidebar.
• Confirm that the subscription and banner visibility status remains the same after app is reinstalled.
• Cancel Subscription in Play store and confirm that the IAP banner is visible in the sidebar and Settings screen.

I completed the cross-platform testing part with Jorge and confirmed that it works as expected.

[wpmobilebot]

You can test the changes on this Pull Request by downloading an installable build (simplenote-android-installable-build-pr1569-d532f4b-018476ce-4a32-4721-9f9a-2d2ece571995.apk), or scanning this QR code:

[danilo04]

I am going to merge this PR since it is working correctly and has just a few things to improve, but they are not blocking.