Unhide BackupTransport#getBackupManagerMonitor()

This needs to be a public API so that BackupTransports can implement it.

Bug: 241760569
Test: N/A
Change-Id: Ic3ac1d3f77bcc1bf5f5babdda7361d949023aa16

core/api/system-current.txt

@@ -1564,6 +1564,7 @@ package android.app.backup {
     method public int finishBackup();
     method public void finishRestore();
     method public android.app.backup.RestoreSet[] getAvailableRestoreSets();
+    method @Nullable public android.app.backup.BackupManagerMonitor getBackupManagerMonitor();
     method public long getBackupQuota(String, boolean);
     method public android.os.IBinder getBinder();
     method public long getCurrentRestoreSet();

core/java/android/app/backup/BackupTransport.java

@@ -661,8 +661,6 @@ public class BackupTransport {
      *
      * <p>Backups requested from outside the framework may pass in a monitor with the request,
      * however backups initiated by the framework will call this method to retrieve one.
-     *
-     * @hide
      */
     @Nullable
     public BackupManagerMonitor getBackupManagerMonitor() {