Code review comment for lp:~sergiusens/ubuntu-touch-session/install_clicks

I guess there are a number of different use cases:
a) package is in app store but we want to preinstall it
b) package is not in app store but we want to preinstall it

and another subcase:
1) package may be removed by end-user
2) package may not be removed by end-user

and what we want to decide is whether we're upgrading/reinstalling packages and whether we do this on every boot.

Now some conclusions:
- it seems clear to me that for a), it doesn't matter whether we update it or not as updates can be delivered from appstore, so we could optionally skip this on boot
- it seems natural that things that are in appstore and hence can be installed after the fact should be uninstallable, so that a) should imply 1)
- it is obvious that in the case of b), we must update it on each boot
- it seems natural that in the case of b), we should not allow removal -- otherwise lets just put it in the appstore

So what I'd propose is followings:
* /usr/share/preinstalled/click and /custom/preinstalled/click contain clicks that aren't in the appstore and must be installed or upgraded automatically and can't be removed (no flag to allow this for now, but should add one)
* /opt/click.u.c is prepopulated in the rootfs with appstore packages that we want to install by default but may remove, but is never upgraded (which I think is what happens with writable pathes?)

We should add a flag for packages that can't be removed (both for appstore and preinstalled packages).