Bug 958341 - Present "motivating service" to /account/create calls. r=rnewman

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https://github.com/mozilla-services/android-sync/commit/f9d27162dcb20e460a0d79f6eb46862bbf3cfb5b
Author: Nick Alexander <nalexander@mozilla.com>
Date:   Tue Aug 26 17:47:33 2014 -0700

    Bug 958341 - Test and review comments.

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https://github.com/mozilla-services/android-sync/commit/7b8289836322faf13c8f99e4f18c85b9dcde7841
Author: Nick Alexander <nalexander@mozilla.com>
Date:   Mon Aug 18 14:53:54 2014 -0700

    Bug 958341 - Present "motivating service" to /account/create calls.

    This is used for metrics and for personalizing the welcome emails.
    Since no email gets sent for /account/login, it is not needed there.