BackupManager#cancelBackups() API

Introduces a cancelBackups() API for
BackupManager. When this function returns,
it is guaranteed that currently running
backup operations won't interact with
the active transport.

Bug: 34760860

Ref: https://docs.google.com/document/d/18MnfwkDfKNtXQBPRmL8vpVgfLgSWJsDja1Nm1QV5hOw/edit#heading=h.9p6yo0wx44k3

Test: GTS tests at ag/1893365

Change-Id: I67f78699bbe763ea71c85937fbc01a5b48694eed
diff --git a/services/core/java/com/android/server/EventLogTags.logtags b/services/core/java/com/android/server/EventLogTags.logtags
index 4559254..2e61550 100644
--- a/services/core/java/com/android/server/EventLogTags.logtags
+++ b/services/core/java/com/android/server/EventLogTags.logtags
@@ -132,6 +132,7 @@
 2843 full_backup_success (Package|3)
 2844 full_restore_package (Package|3)
 2845 full_backup_quota_exceeded (Package|3)
+2846 full_backup_cancelled (Package|3),(Message|3)
 
 2850 backup_transport_lifecycle (Transport|3),(Bound|1|1)