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
Merged-In: I86c830ebd559e31d4576a7606705a056afb064ac
Merged-In: I2011e23c9f894c079ab66cd7cc5c14af572a956d
Merged-In: Ic70901a6aa22a03e97f494e793920ab07a0fd612
Merged-In: I4a7658e1fa6946063ab86a251269413903841ee8
Merged-In: Ia59d45f4e95a536d7982f61ac9c9a1bfc5e8ebb8

(cherry picked from commit cd95278e)

Change-Id: I2e26bef9eddd342f51c02b991632c7ea04fe7e66