Don't validate a finished session

The staging files are deleted when finished.

Bug: 211835862
Test: adb install-multi-package --staged $SOME_TEST_APP
      reboot and confirm the app is installed successfully
      reboot again and check the logcat messages for
      "PackageInstallerSession: Package not valid"
Change-Id: I543003d71dec425fd8e5001449ea9d6958d35864