Modify GCS retry parameters, add more logging

Also add our own retry loop.

Change-Id: I767221df189cabb958504c51d7a04465a1ff23e5
Ensure that Built APK files are valid

If we fail to fetch a file from the Google Cloud Store, we substitute
a zero length file. This change alters this behavior to cause a fatal
error if this happens during an attempt to build a project. This
ensures that zero length assets do not wind up in a built APK file.

Change-Id: I09f841f06dacb413adb49056c73b62f65cddcaa9
Designer property for "visible" now uses checkbox

Replaces pulldown that had text options.

Change-Id: I68a8a202dc24b5638defe9b7df2f3385a830e3c3
Merge branch 'checkboxvisible' of github.com:marksherman/appinventor-sources into checkboxvisible