Fix media playing on LeAudio->A2DP switch

When switching from LeAudio to A2DP on earbuds the AudioDeviceManager
tried to fallback onto the other earbud that was also going to be
disconnected which resulted in an error in LeAudioService which was not
propagated to setFallbackDeviceActive() which then returned true -
meaning that there is a fallback device and so the music can continue
playing (suppressNoisyIntent was set to true).

Bug: 278761210
Test: manual
Tag: #feature
Change-Id: I60b90db411f7f7633372d767cf857530ffedb41e