Add DomainVerificationProxyV2 for new APIs

Implements the request broadcast to the new domain verification agent.

The DomainVerificationService delegates through the
PackageManagerService Handler, to mirror what happened with v1. A
message code is reserved for the domain verification feature and all
feature specific messages are wrapped in that code to isolate them
from other PackageManagerService messages.

Prepares interface for backporting the broadcast to the v1 broadcast,
but that will be done in a future change.

Exempt-From-Owner-Approval: Already approved by owners on main branch

Bug: 163565712

Test: TBD in later change when proxies are combined

Change-Id: If77f0c7facebedadfd00b82120949ed4720e9b58