Fix null pointer exception when set preferred transport

According to bugreport, LE Audio profile is connected but sometimes LocalBluetoothProfileManager#getLeAudioProfile still returns null.

BUG: 375995825
Test: atest CachedBluetoothDeviceTest
Flag: com.android.settingslib.flags.enable_set_preferred_transport_for_le_audio_device
Change-Id: I715411b3992ecdefa96e12a957ecc88ed52277e1