Return a null VCN when the dependencies aren't met

Update the check to return a null for the VcnManager
when the dependencies aren't met.
1) For compat reasons, always return a nonnull VcnManager
   to apps targeting a pre-V SDK.
2) For devices that have pre-V feature flags, return null only
   if FEATURE_TELEPHONY is null.
3) For V+ feature flagged devices, instead check
   FEATURE_TELEPHONY_SUBSCRIPTION.

Bug: 334750038
Test: atest CtsVcnTestCases
Change-Id: I116095bcb2203f8e684e7b735bfe414d6dd459e2