[snap] shell looses track of currently running app when snapd updates it to a newer revision

This has been reported by at least two users for the libreoffice and chromium snaps.

See https://forum.snapcraft.io/t/call-for-testing-libreoffice-5-4-3/2935/6:

  « My LO just refreshed…I had a document open and the Ubuntu Dock seems to think it’s no longer open even though it is (no orange pip)
    Also when I tried to open that file it said I had it open and it was ‘locked for editing’, rather than just refocusing on the window I have open.
    I then got this error when trying to close/save the file:
        saving the document
        file:///home/adam/snap/libreoffice/43/.config/libreoffice/4/user/basic/script.xlc:
        General Error.
        General input/output error.
    Presumably this is because the instance I have open is revision 43 and the new one is 44 that Ubuntu Dock is trying to launch from, but LO should ‘just work’ on refreshes. Given refreshes are effectively silent, ordinary users will be confused as to what’s going on here. How could the experience be made better? How can the system recognize that revision 43 is the one that’s open and therefore not try and launch revision 44 whilst revision 43 is open? »

Apparently, apps that register as a gapplication don't have the issue.