tps-frontend: Race condition in handling of conflicting applications

Reported by user in wb:99ade05510205dedefa3221934d8ac44 against 5.21, reproduced by me in 5.21 and 5.22: when Tor Browser is running and I try to activate the browser bookmarks persistent storage feature, I don't see the dialog that's supposed to ask me to close Tor Browser, and the spinner in tps GUI keeps spinning.

In the logs I see this:

Jan 31 16:04:02 amnesia tpsd[6011]: INFO:feature.py:398: [2] Waiting for the user to terminate processes {'Tor Browser': [10281]}

I cannot reproduce the bug in 5.20, so this regression was introduced recently.