API Review: RestoreSession#restoreSomePackages

1. Rename restoreSome to restorePackages
2. Add @Nullable annotation for BackupObserver, BackupManagerMonitor and @NonNull for 'packages' params
3. Update javadoc to reflect the difference between BackupObserver and BackupManagerMonitor
4. Change 'packages' param from String[] to Set

Bug: 120843781
Test: 1) atest RunBackupFrameworksServicesRoboTests
      2) atest CtsBackupTestCases
      3) atest CtsBackupHostTestCases
      4) atest GtsBackupTestCases
      5) atest GtsBackupHostTestCases

Change-Id: I433739c0b1e62059b10593c233dfb2f461a8630e