Move virtual_camera_service_discovery to virtual_devices namespace

... and config owned by Virtual Device Manager team.

This cl creates new flags.aconfig since the previously used
core/java/android/companion/virtual/flags.aconfig has package
which contains "virtual" C++ keyword so it's not possible to
add cc target due to compilation error in autogenerated code.

Bug: 311459626
Test: atest CameraProviderManagerTest
Change-Id: Id7c60cf4b1eaed2a9b65bcb34f193cab05639953