le_audio: Don't expose already exposed LE Audio active device

Single LE Audio device may be represented as two separate device: input
and output for AudioManager. Since LE Audio serice is registered to add
device in static way it gets calls when input device is add and output
device is add. There is no need to expose twice that the same Bluetooth
device is active.

Tag: #feature
Bug: 271407426
Test: atest GoogleBluetoothInstrumentationTests
Change-Id: I59061171c9501da074a9e24528f9f6bc99174d0c