Cleanup SystemReady in the network stack

The system server (in SystemServer.java) defines the boot
sequence, during which an event called SystemReady happens.
This corresponds to a time when critical system services
that are depended upon by other components in the system
server are ready to handle requests from their dependencies.
Some system services are listening to this event to defer
initializations that depend on the critical services.

Because the network stack is only started after SystemReady,
there is no way any NetworkMonitor may be started before
SystemReady. Remove the associated mechanism.

Fix: 129376083
Test: FrameworksNetTests
Change-Id: I071eeb10d0b7c4f71af6653d322c7b442b2cc7ee