Buy Remote Thermostat For AC (controlled by Android app) ?

IMHO 30 minutes of cooling won't buy you much. Better to set it up a degree or two, ie. 78 instead of 76 and let the A/C keep things inside at a reasonable temperature. If you shut off the A/C completely everything inside will warm up and you will have to undo all of that to be comfortable again.
One problem I have with internet t/s is that if you can access it so can a hacker. I wouldn't want someone to reset it for heat at 100 degrees while we were out.
Like I said IMHO.