Add a second fragment to select the mailbox

After choosing the account, we need to select a mailbox for the shortcut.
In order to replace the fragment, we cannot hardcode it in the XML. Instead,
we need to dynamically create it.

Also, restructure the fragment class and rely on the class to "do the right
thing"

Change-Id: I752ad5bbdf2484332ec2b73852cae74a5d2092fa
10 files changed