Make Remote Submix device attachable

The "Remote Submix In" and "Remote Submix Out" should be
connected/disconnected when using DAP.

This patch:
1. Add REMOTE_SUBMIX to the output device list.
2. Set connection to CONNECTION_VIRTUAL when converting to AIDL.

Flag: EXEMPT bugfix
Bug: 375228511
Bug: 344031109
Test: atest CtsMediaAudioTestCases
Change-Id: Iba3daf6586ad4d03af801d213aac2a35f3b252cf