Fail a staged session when a rollback is already staged (1/n)

1. A rollback session gets the priority. Non-rollback sessions should
   fail no matter they are staged earlier or later.
2. Move checks for commit timestamps into the if block for overlap
   checking. So it will not interfere with rollback checks.
3. Fix StagingManagerTest caused by the change in
   checkNonOverlappingWithStagedSessions().

Bug: 192625695
Test: atest CtsRollbackManagerHostTestCases StagingManagerTest
Change-Id: Id3a0cfaef52d0f8b7b311b28c7bc2bd99e8b0a53