le_audio: Update fallback input device

This CL adds support for update of fallback input device which may be
exposed while broadcasting.

Bug: 329178567
Test: atest LeAudioServiceTest#testSetActiveDevicesFromSameGroup
Test: atest LeAudioBroadcastServiceTest#testUpdateFallbackInputDevice
Flag: Exempt; trivial change covered with unit tests
Change-Id: Ie01099c9ec981eced9b4750c96f6060320d8165d