Fixed NetworkStatsServiceTest and converted it to JUnit4.

Most tests were failing because due to a null NetworkCapabilities.
Example:

1) testNetworkStatsWifi(com.android.server.net.NetworkStatsServiceTest)
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.net.NetworkCapabilities.hasCapability(int)' on a null object reference
	at
	com.android.server.net.NetworkStatsService.updateIfacesLocked(NetworkStatsService.java:983)

BUG: 30839080

(cherry picked from commit 574f370c)

Change-Id: Ie09b2f43cf6ec745e404d5ec98bd0b072d211ea3