New flag: rfcomm_prevent_unnecessary_collisions

Declare rfcomm_prevent_unnecessary_collisions in rfcomm.aconfig

Bug: 349892756
Bug: 348657813
Bug: 315241296
Flag: com.android.bluetooth.flags.rfcomm_prevent_unnecessary_collisions
Test: m com.android.btservices
Change-Id: Ia387122a0c2d69199fad91a675a24c41ce589425