Faster joins: Omit device list updates from partial state rooms in /sync (#15069)

...when lazy loading of members is not enabled. It's weird to notify
a client that another user's device list has changed when the client
doesn't think that they share a room.

Note that when a room is un-partial stated, device list updates are
emitted for every member in that room over /sync.

Signed-off-by: Sean Quah <seanq@matrix.org>