Midnight should mean midnight

...not noon-or-midnight.  Fix the initial fstrim delay to properly refer
to midnight rather than ambiguous 12-hour "zero" so that fstrim job
consideration always starts overnight rather than possibly at the following
day's noon, depending on when the device was booted.

Bug 18076397

Change-Id: Ife3cd5bdeb14c7832b6e2a2de88dcfd1ba731b67