LeAudio/Broadcast: Simplify the broadcast queueing mechanism

The two classes used for queueing broadcast creation and broadcast start
seem to be an overengineered solution which can be replaced with just
two std::optional<> variables, reducing the amount of boilerplate code.

Bug: 322741363
Bug: 316005152
Test: atest --host bluetooth_test_broadcaster
Change-Id: I115120d33e33302be5127218b02b217cfccd1a32