[le audio] use broadcastId instead of Sid to check for local broadcast

Advertising sid is likely to be same for local source and external
source. As a result, external source is unable to be added while local
source is enabled.

This fix use broadcastId instead of advertising Sid to check for local
broadcast source.

Bug: 370267705
Test: manual
Test: atest BassClientServiceTest LeAudioBroadcastServiceTest
Change-Id: I6e969a4f47d4280019c32195c177654e2f713158