Fix handleStreamValidateAndCommit() (5/n)

1. Don't retain finished sessions if isCommitted() is false.
   We don't keep finished sessions if they fail validation to preserve
   the original behavior.
2. Simplify error handling of handleStreamValidateAndCommit()

Bug: 210359798
Test: atest CtsStagedInstallHostTestCases
Change-Id: I713b6b1e88c8978d58c912c8c9fc8b49e4673c82
2 files changed