Save starting/ending times fo scanning to log file

Since Android doesn't fire alarms at the exact time we specify
when alarm interval is very short, i.e. 5 seconds so we need to send
starting & ending times for BLE scanning back to Python client to do
an accurate calculation for the average.

Test: Done
Bug: 38234726
Change-Id: I9106ddce8e358e0a7ef222b51110129804395716
(cherry picked from commit cfdbd7def753412a7914bac42e68f3c9659791f7)