Have NetworkPolicyManagerService create MultipathPolicyTracker

To make connectivity service mainline, this patch makes
MultipathPolicyTracker as a submodule of NetworkPolicyManagerService
to remove the dependencies of ConnectivityService.

Bug: 175015282
Test: FrameworksNetTests
Change-Id: I82a7c62069ffd0683deb2f5ce2f99de120a2a16f