Simplify recoveryTime computation

Stopwatch.stop() returns self, take advantage of that to shorted
the code to compute the formatting string. Also drop toString()
call, as it is implied by string concat.

Change-Id: I4fa645e42fc38b5f9f0775d986fd17aa693725b1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 49f3024ee0a621729e31279f8b2d46051fc003f1)