Fix NPE and turn on logging for device-idle alarms

Test: Manually, using shell commands:
adb shell dumpsys battery unplug
adb shell dumpsys deviceidle force-idle deep
should not cause a crash. Also the output of the logging can be verified
by running:
adb shell dumpsys alarm

Bug: 178687870
Change-Id: Ib085aae73571ff5bfc841a1cc44337c0f7dc64d2