metadata: port .fdroid.yml handling to App dict subclass

This got overlooked in b7fc7f22, then
caught in the ./makebuildserver tests, where it builds Checkey.
1 job for metadata-fixes-and-tests in 20 minutes and 39 seconds (queued for 120 minutes and 26 seconds)
Status Job ID Name Coverage
  Test
passed #11281512
test

00:20:39