fastboot: Use clock_gettime(CLOCK_MONOTONIC) in now()

If the system time changes during the execution of fastboot we might
see some strange output such as:

Sending sparse 'super' 4/20 (254972 KB)            OKAY [-516.263s]

Fix it by changing now() to use clock_gettime(CLOCK_MONOTONIC).
I confirmed that all callers of now() are using it for relative time
and not time since the epoch.

Change-Id: Ic3e9442c2ab21dfb076bfed88915085a183754b0