[pm] fix a system crash caused by cleanUpUser

DeletePackageHelper.removeUnusedPackagesLPw can cause an NPE in
PermissionManagerService if the user state stored in
PermissionManagerService has already been deleted. This CL fixes the
order of things such that we delete the user state in
PermissionManagerService after the apps are deleted.

BUG: 320701119
Test: manual

Change-Id: I0086f79f9c998782226b057dc57877cca8338828