Implement screen state batterystat events

This change defines a new batterystat event for screen state changes and
implements the creation of the events when BatteryStats.noteScreenState
is called. noteScreenState now accepts the display ID and the screen
state reason, and batterystat logs will show screen state event entries
that contain the display ID, state, and reason.

Note that we will follow up with a change to pass the real display ID
from DisplayPowerController to batterystats (right now, we always use
the default display ID 0). We will do that in a separate change since
its impact can potentially extend beyond screen state events (e.g.
batterystats does display-specific data processing).

Bug: 364350206
Flag: com.android.server.power.optimization.battery_stats_screen_state_event
Test: manual
Test: atest PowerStatTests

Change-Id: If29d4fb014364581c900798a23b8cafd10afbe02