Export API of listening for network change events in app2

Currently, due to foreground app will never get blocked by
NetworkPolicyManagerService, so onBlockedStatusChanged cannot be
tested under cts net app.

Thus, listen for network change events in app2 allows subsequent
tests on NetworkCallbacks.

Bug: 118862340
Test: m -j cts
Change-Id: I26ca370fc6ae4dd3f32ce6cf448bae83f3fbfbcc
6 files changed
tree: 5eedfd2a1b9a8bdcb1b84065eab2bdd803e08b6e
  1. tests/