[pm] skip sending PACKAGE_CHANGED broadcast if package has been deleted

ag/15926042 introduced a regression when the broadcast could still be
sent if the package has been deleted. It's unlikely that this caused
b/231318806 (where the broadcast has already been sent out before the
package was deleted), but this still needs to be fixed.

+ Also remove an unnessary handler.post() because it is already inside
  the post.

BUG: 233612768
Test: presubmit
Change-Id: I25d906575a61d051df87a8985a5bb754642cc92b