Fix CDMA conference active call inference

The special logic here is only needed for CDMA calls, since they don't
let us see what the active call is directly. But the rest of the logic
should be used for all conference calls. So move the CDMA-specific check
to gate just the relevant part.

Bug: 234629992
Test: manual + BluetoothInCallServiceTest
Tag: #stability

Change-Id: I43e50d5e672ec0cb9ab41445f79d60b9e5a567cd