Don't ostensibly back off fstrim but then do it anyway

When the mount service isn't up yet, we schedule a retry in the near
future, but then mistakenly go ahead and try (unsuccessfully) to do the
fstrim immediately anyway.  Don't do that.  Furthermore, when in this
situation make sure to preserve knowledge of any completion callback
that may be expecting to hear back about the [eventual] fstrim operation.

Change-Id: Ic7cba37b4f850540f6e448168be0c42418039b01