UsageStats tracking of screen usage.

1. Add UsageStats Event types:
ACTIVITY_RESUMED is synonym to existing MOVE_TO_FOREGROUND.
ACTIVITY_PAUSED is synonym to existing MOVE_TO_BACKGROUND.
ACTIVITY_STOPPED when an activity becomes invisible on the UI.
2. In UsageStats.java, add API getLastTimeVisible() to report last time the
app is visible (ACTIVITY_RESUMED or ACTIVITY_PAUSED), add API getTotalTimeVisible()
to report total time the app is visible.
The existing API getLastTimeUsed() can report last time the app is in
foreground (AKA have focus).
The existing API getTotalTimeInForeground() can report total time the
app is in foreground (AKA have focus).
3. UsageStats.getTotalTimeVisible() can report screen usage for
split-screen mode and picture-in-picture mode.
4. Because in the same package, activity can be instantiated multiple times,
In UsageEvents.Event class, add a member mInstaceId for activity's
instance ID, add interface getInstanceId() to retrieve the instance ID.

Bug: 112002260
Test: frameworks/base/services/tests/servicestests/src/com/android/server/usage/UsageStatsDatabaseTest.java
atest frameworks/base/core/tests/coretests/src/android/app/usage/UsageStatsTest.java

Change-Id: Ibcef2488e9620804c9f9220b027f976e8fa0c98b