Dehydrated devices: Add some stuff to the crypto dev tools

[richvdh]

We should consider adding some information about the state of dehydrated devices to the crypto dev tools (see #29073), like:

• do I have a dehydrated device? - same logic as Device dehydration: users should be able to see their own dehydrated device(s) #29264 is there a dehydrated device in the devices list?
• does the key for that device exist in 4S?
• do I have the key to the dehydrated device? - do we have the key cached?
• when is the dehydrated device scheduled to be rotated?

(Context: dehydrated devices are created often, e.g. when you refresh the tab, and each time one is created, the old one is deleted.)

Tasks:

• js-sdk should remember the timer it set for rotation of the device so it can display it
• Expose this information, which is known inside js-sdk, to element web
• UI to display the above info (no design needed because this is a dev tool)

[richvdh]

For reference: currently, if you have a dehydrated device, that is shown under "Secure Backup" (of all things) on the Security & Privacy tab:

However, we're cleaning up that tab as part of #26488, and the new designs don't include anything about dehydrated devices.