blueooth/audio/aidl/default: Fix return code in startSession

BluetoothAudioProvider::startSession() currently doesn't use the
return code from onSessionReady() and that basically hides all
exceptions return by the child implementation, e.g
A2dpSoftwareAudioProvider::onSessionReady(). This commit fixed
that so the errors are propagated to the binder service.

Bug: 399554337
Test: m android.hardware.bluetooth.audio-V1-impl
Change-Id: I37be56769ceb00caacf0085eef3033469be2a9a8