Early exit if we don't have a package on record

In NMR1, the ability to silently uninstall orphaned packages was added.
This code path added a new throw from the uninstall flow if the package
didn't exist and resulted in uninstall of a non-existant package
printing a stack trace instead of DELETE_FAILED_INTERNAL_ERROR as it did
in prior versions. This change simply checks for the existance of the
package and fails early when no such package exist. Also moves all calls
to the observer to a post to the handler.

Change-Id: I89d1d5002ef2f37b65a6910fb366ff4a68070e6d
Fixes: 33230110
Test: unisntalled missing package