Stop using IIpConnectivityMetrics in ConnectivityService.

Currently, ConnectivityService calls the IpConnectivityMetrics
service class directly to log default network events. This is
incompatible with ConnectivityService being in a mainline module.
Replace direct access to IIpConnectivityMetrics with public
methods in IpConnectivityLog, which is @SystemApi class.

The new methods are not yet @SystemApi, but they can be made so
if desired. Alternatively, these metrics could be deleted.

Also remove the IpConectivityMetrics service from the
service-connectivity JAR, and go back to starting it from
SystemServer.java, which is what was happening a few hours ago
before aosp/1542626 was merged.

Test: builds, boots
Test: atest FrameworksNetTests
Test: "dumpsys connmetrics" shows events, including default network events
Change-Id: I9d6147d93590363a2f8f83f39f05c03d001b4851