Fix issue # 3227963: SecurityException: Neither user 10023 nor...

...current process has android.permission.WAKE_LOCK

When updating a system app, we would actually uninstall the package
of the system app, which also meant removing its uid...!  It was just
luck that we would get the same uid when installing the update after
that.  During that time, if anyone tried to do anything related to
that uid, it would be unknown.

This change tweaks how we go about replacing system apps by making
it more like normal apps -- to make this work, if we need to disable
the system app, we generate a new PackageSetting from the current
system app and replace it into our data structures, so we can update
that without trashing the current correct information about the (still
actually there) system app.

Also fixed a problem where we were not killing the currently running
app before installing, like we do when updating a normal application.

And fixed a problem where we were not deleting the /data .apk when
uninstalling a system app update.

And added a new option to the "pm" command to clear the data associated
with an app.

Change-Id: I0e879677849aa42950a3c360bf78ad820e87674b