BatteryStats: Add debug logging to narrow down slow camera launch

BatteryStats is on the hotpath when the screen is turning on.
Double tap to launch camera may be impacted on some dogfood devices.
Adds logging (that will be removed before release) to help narrow
down the cases when this happens.

Bug: 36201811
Test: builds/manual
Change-Id: I8080ac09b21269237eda6215d73199676de2005a