Fix NPE when trying to get input device for attributes.

1. Return error when remote submix device is null. Remote submix device
could be null in some case, such as device address error. Returning
error to avoid native crash.
2. Unregister policy mix if set device connection fails. In that case,
get input device could fail when policy mix is not found.

Bug: 131906484
Test: Manually
Change-Id: I03ab2bf217341b672c14adfba96d92ad9b453b01