force re-syncing GCM keys

I don't think this is a top priority suggestion, but it would be nice to have nonetheless for the times a reset of the GCM keys is needed. I'm not sure why it happens, but for me it happens when I restore data for Join using TiBackup after a reset of my device. in Join's Diagnostics I would see that "Receive Push from server", "Receive Push with direct GCM Key", and "Receive push without going through Join's server" all fail despite refreshing the Devices list. seems like wiping Join's data is the only way to get everything working, however there are times when I'm lucky enough that Join offers the option to get new GCM keys otherwise I'm stuck. would be nice to have an option in Join that will allow one to force a re-sync of the GCM keys on the Android app to avoid this in the future.

failing the Diagnostics did also happen on another device that I never restored data for, but for that device I was also lucky enough that I received the message to re-sync the GCM keys while in Join. not sure what would have happened if I accidentally pressed no at that time... hence having the option to force a re-sync would be nice.