Fix spurious restore session timeouts

The restore-session idle timeout should not be ticking while we're
doing legitimate restore work.  We now explicitly stop the timeout
ticker [a delayed message on our handler thread] whenever we undertake
a valid restore operation.  The timer is already correctly resumed
when restore operations conclude.

(In practice we need to suspend the timeout tracking at exactly those
times when we're entering the wakelock-protected restore flow.  The
timeout is reestablished when the wakelock is released; this part
is already in the code.)

Bug 17990544

Change-Id: I7318020ce30fd9c35bc3a644f8c101fd3d063c8b