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Previously this would fail if the framework wasn't stopped. The failure
would then stop full disk encryption. The fact that the unmount worked,
however, would then stop the second attempt from achieving anything.

Fix in line with current retry philosophy

We still need to figure out why Devmapper::destroy() fails at first.

Bug: 17301843
Change-Id: I405a36c832ccdebf2d904bef77f15eea174a6bfb