Skip post install if package was deleted

This change skips the majority of handlePackagePostInstall if the
package being handled is no longer in settings. This can happen if an
uninstall is issued post commit, but before the post install steps are
posted to the package handler. This will be addressed more completely in
a later change.

Fixes: 153131678
Bug: 153456530
Test: Device boots, installs complete without issue; this is a difficult to reproduce race condition.
Change-Id: I448a6ac11f6dd6f1f30859e05ab971bae0b5d5e9