Fix: ConnectivityManager is initialized with a wrong context

ActivityThread#setHttpProxy uses a system context to get
a ConnectivityManager instance. If it's not already cached, the
system service registry will cache this instance initialized
with the system context, and subsequent calls to get this service
will return the bad instance.
This patch uses the application context instead, unless it's
not available for some unlikely reason.

A possible symptom of this issue is to have Bluetooth tethering
failing with a security exception, because it's requested by
the "android" package without system uid.

Bug: 73572062
Test: manual
Change-Id: I745707e4ffe564f6d201ec259fa6b5ee32fbeda6