Replace redundant error code with a specific one

As discussed on b/163039465#comment3, it's not desirable to have two
distinct catch-all error bucket, where the difference is very subtle.

Opting to use STAGED_SESSION_CONFLICT which is more specific, but not
too specific. We can use this error is any scenarios where a staged
session interferes with installation of another staged session and
we have to fail one of them, e.g a staged session is blocking a
rollback or rollback will be causing API downgrade which will break
another staged session (b/163734200).

Bug: 163039465
Test: atest atest RollbackManagerHostTest#testRollbackFailsBlockingSessions
Test: atest StagedInstallTest
Change-Id: Ia05350024093cd537e8ced5ea02af5853620d6ef