Don't crash ConnectivityService if the network stack crashes.

When the network stack crashes, the system will rebind to it.
Existing references are no longer useful (they just throw
RemoteException) but if the system is still up, then the user
can at least recover the situation by taking actions such as
going into airplane mode, toggling wifi, etc.

This CL stops ConnectivityService from crashing the system when
it cannot talk to NetworkMonitor. This is arguably better than
crashing the system, because crashing the system is disruptive
and carries the serious risk of a bootloop from which it is not
possible to recover.

NetworkStackClient already contains code to crash the system
when the network stack crashes. This change help ensure that
if a crash occurs, it is the result of an explicit decision by
that code instead of an unchecked exception in one of the callers
of the network stack.

Bug: 133725814
Test: builds, boots
Test: atest FrameworksNetTests NetworkStackTests
Change-Id: Ib9a15fececd8579fc5b139fe0341275a45512e0f