Include VCN in apex based on the build system flag

This patch includes VCN lib to apex jars when the build system flag
RELEASE_MOVE_VCN_TO_MAINLINE is enabled.

This patch also connects the build system flag to the
soong_config_bool_variable "is_vcn_in_mainline". The
"is_vcn_in_mainline" variable will be used in Android.bp files to
conditionally include VCN in either the Tethering module or
the platform.

Bug: 375213246
Test: flag enabled: VCN libs are in the tethering module
      flag disabled: VCN libs are in the platform
Flag: RELEASE_MOVE_VCN_TO_MAINLINE
Change-Id: I6240c0222fab042a8cf1794632358bf855903bba