interfaces: make upower-observe implicit on core if /usr/libexec/upowerd exists

[robert-ancell]

This change is similar to one done for polkit.

We want to be able to use the upower-observe interface on Ubuntu Core Desktop, so need an implicit system slot to be present there. This PR takes the approach of providing an implicit slot if the executable /usr/libexec/upowerd exists. This should be true for Core Desktop, and false for regular Core.

Note that there used to be a upower snap, but this was deprecated.