[le audio] Broadcast reset saved fallback active device

Followup CL to address comment from aosp/2908958. We'd need to reset
this device at service start() and stop().

Bug: 319939252
Bug: 316005152
Test: atest LeAudioServiceTest
Test: manual test with new UI and switch active device during broadcast
Change-Id: I525b061a0a87bbdb1e859c4af648c70295c69fa7