BackupManager shouldn't clear app's data during restore if agent wasn't invoked yet

Bug: 123738850
Test: 1) atest RunBackupFrameworksServicesRoboTests
      2) atest CtsBackupTestCases
      3) atest CtsBackupHostTestCases
      4) atest GtsBackupTestCases
      5) atest GtsBackupHostTestCases
      6) Manual: Force exceptions before/after agent was called, verify
                 the new behavior.

Change-Id: Ia9ddea2939fc6d72dd9bc060b0c47bc8da2b868d