UsageStats DEVICE_SHUTDOWN event.

DEVICE_SHUTDOWN event is used to close all open usage events that do
not have matching closing event when device is shut down. For example,
ACTIVITY_RESUMED or FOREGROUND_SERVICE_START are open events, the
DEVICE_SHUTDOWN event will close the usage session of the open events.

At orderly shutdown like selecting Power Off or Restart after pressing
power button, a DEVICE_SHUTDOWN event is sent to UsageStats.
UsageStats persists UsageStatsDatabase to disk immediately.

When power button is pressed for 3.5 seconds (configured by
config_veryLongPressTimeout in config.xml). A DEVICE_SHUTDOWN
event is sent to UsageStats. UsageStats persists UsageStatsDatabase
to disk immediately.

This is the mechanism that we do not lose UsageStats data when the
device is shut down.

When the device boots up, if the last event is not
DEVICE_SHUTDOWN, we add a DEVICE_SHUTDOWN with timestamp set to be the last
time database file is persisted. This is to handle the case device
shutdown abruptly due to power drained or cold temperature.

Bug: 111464278
Test: atest UsageStatsTest.java
Change-Id: I1e88063ba71d09042d02c6deb9f07d8581a15c30