Eliminate race condition around backup completion + resumption

Ensure that the callback always sees the current-operation state in sync
with the various other bits of internal backup-operation state.  Previously
only the current-operation state was managed inside the critical section;
this resulted in a slim race window where a callback could see an ongoing
operation as still valid, but after the internal state on which that
operation depended had already been cleared.

Bug 17931760

Change-Id: Ia032668e7a9d22f1029c57fc98db9e86484d5719