Merge cleanStageDir() and destroyInternal() (1/n)

Now that we have refactored staged installation, we no longer
destroy staged session on commit success. It is now possible to
call destroyInternal() when we really want staged sessions to be
cleaned up.

Bug: 173194203
Test: atest CtsAtomicInstallTestCases
Test: atest com.android.server.pm.test.PackageInstallerSessionTest
Change-Id: I38b3d27d13940545f7db17d542137a3ed57a996d