Uninstaller: Background uninstalls for all users

If the caller requests an uninstall but does not ask for the result we
we execute the result in the background and show a notifiation on
failure.

Currently this is using the offical package install API that used to be
restricted to a single user. As it is not clear how we extend the
official package install API, I am switiching to a internal method
which should be close in behavior to the final official behavior. This
unblock other changes.

This means that we can now execute uninstallations for all users or for
different profiles of the same users with the background workflow.

This also fixes the error string shown when the blocking user is the
system. It is now the same as in N.

Test: Uninstalled across user boundaries.
Change-Id: I819ffa00429e720d805cbc080fbf7ec7dc0e92fc