Bug 934449 - [Messaging] Tapping on '+' to add a contact to a message shows the contact list with the keyboard

https://bugzilla.mozilla.org/show_bug.cgi?id=934449

- ThreadUI.requestContact sets Recipients.View.isObscured to true before creating MozActivity
- Set Recipients.View.isObscured to false when activity.onsuccess or activity.onerror are invoked.

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>