Add crypto.willUpdateDevices event and make getStoredDevices/getStoredDevicesForUser synchronous
Add an event fired before devices are updated, so apps can monitor
changes in devices by observing the state before and after.
BREAKING CHANGE:
Make getStoredDevices/getStoredDevicesForUser synchronous so they
can safely be called from the event listener without racing with
the data being updated. They didn't do any async work so didn't need
to be async.
Add doc for crypto.devicesUpdated