Implement ConnectivityService TODO and fix many race conditions

This patch implements an outstanding TODO in ConnectivityService to add
synchronization over the map of network request ids to network agent
info objects.

This structure is accessed from multiple threads:
 - Binder thread on public aidl methods, most notably via
   getDefaultNetwork().
 - Internal handler.

This leads to many race conditions that can crash the system server and
reboot the phone if getDefaultNetwork() is called on a Binder thread to
service a public ConnectivityManager api while the default network state
is being updated on the internal handler after losing the default
network.

Bug: 65911184
Test: runtest frameworks-net
Change-Id: I86c830ebd559e31d4576a7606705a056afb064ac