init: wait_for_file use smaller time resolution

As 1s timeout can be requested, using second resolution time to check
for timeout prevent from being accurate on the actual time we wait.
Use available gettime_ns instead.

Bug: 21374269
Change-Id: I8be1b69e02beacc7196427f97855e036addf54d1
Signed-off-by: Thierry Strudel <tstrudel@google.com>