Bots don't have custom bot config before they call /handshake, and thus they use only default bot config to configure dimensions, which often produces "wrong" results.

This is seen most clearly in Android bots. They report themselves as Ubuntu-16.04 right after restart: https://screenshot.googleplex.com/15thCB3COhH.png 

Since they initialize their android guts after the handshake, these wrong dimensions stay for quite a while (~1 min). Eventually the poll with correct dimensions.

This doesn't affect task reaping, since /poll always happens with correct dimensions, but it still has negative effects for anything that queries bots per dimensions (monitoring counters, botmap updater script, "N bots can run this task"). It also spoils BotTaskDimensions cache each time bots restart.

(You can see how botmap updater script detects this flapping between Ubuntu and Android e.g here https://chrome-internal.googlesource.com/infradata/hosts/+/70908d98e44107852b37c0a03fa1a849dcea54e3. In fact most of the changes to hostmap is this flapping).

I propose we do not write entites relatated to bot dimensions in /handshake and do it only in /poll.