Implement TestNetworkService

This change adds the relevant JNI implementations, as well as the
implementations and NetworkAgent tracking code in the TestNetworkService.

TestNetworkService (And Manager) is designed for use exclusively in
testing code, allowing for the creation of TUN and IPsec backed networks
in test code. Specifically, this allows for testing of components such
as IP, DHCP, or DNS clients, as well as kernel features such as IPsec.

Access to the TestNetworkService will be conditioned upon the
MANAGE_TEST_NETWORKS permission that will be granted only to the Shell.
CTS will use UiAutomation.adoptShellPermissionIdentity() to gain the
ability to use this service.

Bug: 72950854
Test: CTS tests passing
Change-Id: Ie66ba631a548b5f9c6b5ed0797582f86688debe5