Fixing issue in current and max duration calculations.

It looks like one operation was done out of order and some of the times used in
the calculations were leading to incorrect results.

BUG: 31023263
Test: bit FrameworksCoreTests:com.android.internal.os.BatteryStatsDurationTimerTest

Change-Id: I417cc28c5a55748067b6c7f682a66fe3dbc09f09