Add UsageStats events for foreground service start/stop.

1. Send FOREGROUND_SERVICE_START event when foreground service starts.
Send FOREGROUND_SERVICE_STOP event when foreground service stops.
2. One app can multiple foreground services and multiple services can be
started. Because this, in UsageStats, change mLastForegroundEvent to
className to event map, do this for both activity and foreground
service. Change UsageStatsProto and UsageStatsXmlV1 to support this
change.
3. Add more test cases in UsageStatsTest.java.

Test: start music player which is foreground service, observce these
two events when start play and pause play.

Change-Id: I3dc14f5b73cc114a53b8c51f90d3011d9ace35ac
Bug: 112002260
Test: atest UsageStatsTest#testForegroundService
atest frameworks/base/services/tests/servicestests/src/com/android/server/usage/UsageStatsDatabaseTest.java
atest frameworks/base/core/tests/coretests/src/android/app/usage/UsageStatsTest.java