Migrate NetworkSecurityConfigTest to JUnit 4

This change updates the `NetworkSecurityConfigTest` from JUnit 3 to JUnit4.

This is required for testing Aconfig flag changes, notably the one deprecating the usesCleartextTraffic flag ( http://ag/35312110 ).

Bug: 415007211
Flag: TEST_ONLY
Change-Id: I4b0e15bb79cdc5a55c4a918a08f6bb7aab8c413c