Add required flag to registerReceiver call in NetworkMonitor

Android T adds support to allow a runtime receiver to be registered as
not exported, but to ensure apps can take advantage of this, calls to
registerReceiver must specify a flag indicating whether the receiver
should be exported for apps targeting T+ that are registering for
non-system broadcasts. This commit adds the RECEIVER_NOT_EXPORTED
flag to the call to registerReceiver in NetworkMonitor since this
action is only broadcast via a PendingIntent created by the system.

Note, this change will only affect devices running T; prior to T,
there was no way to mark a dynamic receiver as unexported, so this
change uses the platform's default of 0 as the value for the flags
passed to Context#registerReceiver on devices running Q through S.
There's also basic testing in ACTS under tools/test/connectivity/
acts_tests/tests/google/net/CaptivePortalTest.py; while this is not
a blocking test, it will report if there are any issues with this
flag in T.

Bug: 161145287
Fixes: 216213421
Test: atest NetworkMonitorTest
Change-Id: Id8d623fde0715529afa68b3837e81fbdb1a66d12