Improve broadcast supported check to support private audio sharing

For private audio sharing, the device might have no actively connected LEA devices.
Need to improve the broadcast support check for callback unregister and
active broadcasting cases.

Tag: #bug
Bug: 294618457
Test: atest MediaOutputDialogTest MediaOutputBaseDialogTest
Test: manual Broadcast with two connected pair of LE Audio devices
Change-Id: Ibf939a09a2d376b7aa834ccb2f7c96df4db08b5c