[DK2-0]Create AutomaticOnOffKeepaliveTracker

Add a new AutomaticOnOffKeepaliveTracker class between
ConnectivityService and KeepaliveTracker to handle the automatic
on/off keepalive. This commit only creates this new class and
move the TCP polling code to the new class as a preparation for
the following commit.

The original test file was created for testing the TCP polling
mechanism, so rename it to match the new class.

Bug: 259000745
Test: m ; atest FrameworksNetTests
Change-Id: I1b229f906283c0f5ef7a3efdb0572fcbfc5df72b
4 files changed