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Some devices can take up to a few minutes to do fstrim.  If done
in the same thread as the rest of the vold command listener, then
vold is blocked from responding to any other commands until the
trim is done.  So create a thread to do the work, and return
immediately.

bug: 8688454

Change-Id: I780baae03ba7de2d3e805c3e9f103ec03be84c47