Good idea.
But it's a can of worms.
Perhaps something like this might make sense:
a) If I go to the android market, read about the application, decide to install, I'd rather it not be blocked unless it contains malware. Avast could ask 'do you really want to install', I'd say YES, and would like to see that in some log.
b) If I did not make an installation request, and not sure whether avast could detect that I did not, but a browser or some other application wants to install something behind my back, then I'd like to see it blocked, logged and alerted.