Don't subtract duration twice in dummy camera grab
(cherry picked from commit f6bd8dec2fa79cf660fda086d27027ecb2b3bc4f)
Fix sleep in dummy camera grab

time.sleep doesn't accept pint quantities, so we have to get
the magnitude of the time and then can do the sleep.
(cherry picked from commit b10bf51bdf03e25b2473a5f351d5a2a2817d18a1)