Element-R: Replace `CryptoBackend.checkDeviceTrust`

[richvdh]

(currently stubbed).

I think it's a matter of calling OlmMachine.getDevice and then mapping the result back into a DeviceTrustLevel. Expose it via CryptoApi. Use it in places in the react-sdk where we currently call CrossSigningInfo.checkDeviceTrust directly.

[richvdh]

turns out we have to replace it, rather than implement it, because we need it to be async.