Allow a birthday height to be entered in the Keystone wallet setup flow.

[nuttycom]

At present, there's no way to enter a birthday height, which means that existing funds controlled by the Keystone device will not be detected if re-enrolling a device after deleting and reinstalling the app.

Some funds are currently incidentally recovered due to the wallet's default behavior of rescanning the ranges covered by the chain-tip shard, but this is not a reliable mechanism for recovering funds and will miss any funds received before the chain-tip shard range.