Start BackupManagerConstants content observer

BackupManagerConstants has a content observer that is meant to be
notified whenever backup settings change. However, that observer is
never registered.

Bug: 71622864
Test: cts-tradefed run cts-dev --module CtsBackupHostTestCase
Change-Id: Icbd90ef3af83dd2f29e26a5d787505bcf5681154

services/backup/java/com/android/server/backup/BackupManagerConstants.java

@@ -85,9 +85,6 @@ class BackupManagerConstants extends ContentObserver {
         super(handler);
         mResolver = resolver;
         updateSettings();
-    }
-
-    public void start() {
         mResolver.registerContentObserver(
                 Settings.Secure.getUriFor(Settings.Secure.BACKUP_MANAGER_CONSTANTS), false, this);
     }

services/robotests/src/com/android/server/backup/BackupManagerConstantsTest.java

@@ -57,7 +57,6 @@ public class BackupManagerConstantsTest {
 
         final BackupManagerConstants constants =
                 new BackupManagerConstants(handler, context.getContentResolver());
-        constants.start();
 
         assertThat(constants.getKeyValueBackupIntervalMilliseconds())
                 .isEqualTo(4 * AlarmManager.INTERVAL_HOUR);
@@ -89,7 +88,6 @@ public class BackupManagerConstantsTest {
 
         final BackupManagerConstants constants =
                 new BackupManagerConstants(handler, context.getContentResolver());
-        constants.start();
 
         assertThat(constants.getBackupFinishedNotificationReceivers())
                 .isEqualTo(new String[] {PACKAGE_NAME, ANOTHER_PACKAGE_NAME});