Fix: ConnectivityManager methods crashing with SecurityException

Starting in R, some methods in ConnectivityManager like
getNetworkCapabilities started passing the package name from
the context stored in CM to check that the package is really
whom it pretends to be. Unfortunately, in some cases, the
context contains package "android" for an app, and since the
app is not the system, the check fails and crashes the app.

It seems the culprit is updateHttpProxy, which is called by
ProcessList when the PROXY_CHANGE_ACTION broadcast is sent.
If this happens to run between the time the process is created
and the activity thread is "bound", then the mInitialApplication
member is not set, and updateHttpProxy uses a system context.
Since ConnectivityManager caches the context forever in a
static, this leads to subsequent legitimate calls crashing.

Setting the proxy can be deffered until such a time that the
app is bound, as it can't run any code before then. The member
is never reset to null, so it's guaranteed to be non-null at
bind time.

An alternative would be to post a runnable on the handler
thread if the member is null to try again later. This
could however run the lambda a considerable number of times
as binding can be delayed, and risks causing an infinite loop
if some invariants are changed in the future.

See also b/73572062 and ag/4056059

Bug: 155549446
Bug: 189360509
Test: ActivityThreadTest FrameworksNetTests NetworkStackTests
Test: Manually set a proxy, observe the broadcast being sent and
      apps not crashing
Change-Id: I956f76be2e0a1a675576511fb394d7ed4354b28a
(cherry picked from commit b0d13e29515d5b7c82daed7533b78ac57e46bd93)