Added tron metrics for shutdown time

The metrics are persisted in /data/system/shutdown-metrics.txt file
property and logged after reboot. MetricsLogger API cannot be directly
used during shutdown since event log is cleared on reboot/power off.

Test: manual reboot/shutdown and verify that metrics are logged
Bug: 64815357
Change-Id: I7fa1af8be58a80a3d5bae26d55c1ed40bf98f4c0