[lib] make signedDeviceList in DEVICE_LIST_UPDATED message optional

Summary:
To standardize sending updates for all Identity auth (legacy/QR-code) we need to remove signedDeviceList. However, @bartek added additional check for broadcasted and Identity device list equality and we want to keep it for now.

Depends on D12330

Test Plan: Flow - functionality tested later in the stack.

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat, bartek

Differential Revision: https://phab.comm.dev/D12347