[le audio broadcast] Handle STOP message in enabling state

Currently if broadcaster is creating BIG in enabling state, we ignored
the STOP message, and this will cause us stuck in an incorrect state.
This CL handles this case by stopping the broadcast and cleaning up the states
properly.

For late BIG event
1. Create BIG, in enabling state
2. STOP received, switch to stopping state, but NO disable announcements
3. When the prev BIG create complete, in HandleHciEvent -> stopping will terminate BIG
4. When BIG terminated, HandleHciEvent -> HCI_BLE_TERM_BIG_CPL_EVT -> DisableAnnouncement()
5. OnEnableAnnouncement -> STOPPED state

For late ISO event
1. Create BIG, in enabling state, and BIG setup completed, start setup
   ISO
2. STOP received, with my change, in stopping state, but NO disable announcements
3. When all ISO complete, tear down all ISO
4. When all ISO teared down, OnRemoveIsoDataPath -> terminate BIG
5. HandleHciEvent -> HCI_BLE_TERM_BIG_CPL_EVT -> DisableAnnouncement()
5. OnEnableAnnouncement -> STOPPED state

Bug: 398723382
Flag: EXEMPT, trivial fix covered by unit tests
Test: atest bluetooth_test_broadcaster_state_machine --host --no-bazel-mode
Change-Id: I0a8d1ae0c774f48080f635503c7cbc071ed5d36e