Fix broken testCaptivePortalApp

This was broken by aosp/1657763, only on eng builds because
of a wtf. The root cause of the issue is that the test is
acquiring MAINLINE_NETWORK_STACK after it registers a callback
but before in unregisters it. After aosp/1657763, the objects
keeping track of callbacks are separate for processes with and
without MAINLINE_NETWORK_STACK, which means the removal is
looked up in a different counter.

Solve this by having the NRI remember the relevant counter
keeping track of it.

Bug: none
Test: testCaptivePortalApp
Change-Id: I566a28522fa68be9cc2bdb4624b588b4f01d3b7e