Conditionally remove VCN jars from non-updatable platform

When the build system flag RELEASE_MOVE_VCN_TO_MAINLINE is true,
VCN framework and service jars will be built into Tethering module.
Otherwise, VCN framework jar will be directly installed on the
device and the service jar will be included into the platform
services.jar

Bug: 375213246
Test: atest CtsVcnTestCases && atest FrameworksVcnTests; verified
      with the flag on and off
Flag: RELEASE_MOVE_VCN_TO_MAINLINE
Change-Id: If2651f1ede34220172c419f8bf4afb0fe09c0789