Allow SystemServer to create VCN service with a service initializer

We need to use "startService" to load the service via reflection
because the followup CLs will use a build system flag to control
whether the service is in Tethering module or the platform. Using
reflection ensures that in both case the system can build

Bug: 366598445
Test: atest FrameworksVcnTests && atest CtsVcnTestCases
Flag: EXEMPT no functional change
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d6ecfc7de6ca836fa2eb92e05e5abbd3c7095cd7)

Change-Id: I165e83135c557619518257eb79a00b9ce5802c76
Merged-In: I165e83135c557619518257eb79a00b9ce5802c76