Change VCN flag container to tethering

VCN will be moved to Tetherig module eventually. This patch updates
the VCN flag host container to the module and updates the framework.jar
to access the flags via "exported" mode.

Followup CLs will move VCN to a seperate java_sdk_library and the
aconfig lib will be removed from
framework-minus-apex-aconfig-declarations and moved to that
java_sdk_library. A build system flag will control whether the VCN
will be in platform or the tethering module.

Bug: 376339506
Test: presubmit
Flag: EXEMPT flag container value change
Change-Id: I0434abce40dbadeec20f373f46484c41a08947ff