Clarify docs for BackupTransport.initializeDevice()

Explicitly call out that even if the backend is in an undefined state
(such as no authenticated account, so there is by definition no work
that can be done here), the transport should return TRANSPORT_OK in
response to initializeDevice().  Error returns from that method must
only be used to indicate that the initialize should be *retried*.

Bug 21136957

Change-Id: I41ad115b92c16092b4d931d9c554a44b77945359