Adjust wifi idle times to fit in system time period

The battery stats time period calculation is done using a
different clock source compared to the wifi chip's on time calculation. So,
account for skews between the two by adjusting the idle time reported by
wifi chip to fit into the system time calculation.

BUG: 28628799
Change-Id: I3110c53317f3246e26fdc3731f7be0a3d9c8e229
TEST: Compiles & manual verification