Call dispatchSessionFinished() on a staged session when finished (3/n)

* Improve consistency between staged and non-staged sessions.
* It is up to the PackageInstallerService policy to decide how
  to retain finished sessions for debugging purposes.
* onSessionFinished() now keeps finished staged sessions if they are
  not explicitly abandoned by the user (isDestroyed() is true)
  to preserve the original behavior.

Bug: 210359798
Test: atest StagingManagerTest \
            PackageInstallerSessionTest \
            CtsStagedInstallHostTestCases

Change-Id: I87f9c2b562206e20d96f212865df2320d6e3c627