[le audio broadcast] Avoid unexpected device activation when starting broadcast

Before starting broadcast session, current active device will be inactivated first. ActiveDeviceManager will set the other device to be active once the previous device is inactivated.

When broadcast is starting, we should avoid such unexpected device activation by checking both isBroadcastActive() -> broadcast active and isBroadcastReadyToBeReActivated() -> broadcast to be activated.

With this change, the sequence is
1. App requests to start broadcast with 2 group, A(active fallback) & B
2. Stack will set A to be inactive
3. ActiveDeviceManager check isBroadcastingAudio(), isBroadcastActive() -> false and isBroadcastReadyToBeReActivated() -> true, and skip switching active device to B
4. Once A is inactivated, LEA service will continue to start broadcast, isBroadcastActive() -> true.

Bug: 399677373
Flag: EXEMPT, trivial fix covered by unit tests
Test: atest ActiveDeviceManagerTest
Change-Id: I04a68a78743597030bab91fafa0890b3f5b6c78d