Use safe_union correctly

As described in source.android.com/devices/architecture/hidl/safe_union
value must be assigned through generated getter, rather than assigning it directly.

Test: vts-tradefed run vts -m VtsHalBluetoothAudioV2_1TargetTest
Sponsor: jpawlowski@
Bug: 150670922
Change-Id: Iefc7ee98163ec2b3387fc01c3d69c1dcd15e94bc