NetworkStackClient: Refactor network stack process interaction

Refactor NetworkStackClient class to move the module service binding &
network stack process death monitoring to a separate class. This class
will only instantiated in the SystemServer process.
The new class |SystemServerToNetworkStackConnector| will be used from
the client classes corresponding to each module running on the network
stack process (NetworkStackClient, WifiStackClient, etc)

This has 2 main advantages:
a) Reduces code duplication (Otherwise the various Client classes need
to replicate the service bindding & process death monitoring).
b) Central crash recovery for the network stack process (Otherwise the
various Client classes will trigger multiple recovery for a single
network stack process crash).

Bug: 135679762
Test: Device boots up & connects to wifi networks.
Change-Id: I673581b0067b9a3f72dd68a3ab622c18183ebd2e
Merged-In: I673581b0067b9a3f72dd68a3ab622c18183ebd2e
(cherry picked from commit 7e6f5f5e)