Fix NetworkStatsAccessTest

This patch fixes NetworkStatsAccessTest by taking into account
DEVICESUMMARY which was added in commit
3a8b343c.

Doing $ git revert 3a8b343c shows that
the existing assertions of NetworkStatsAccessTest passed before that
commit.

This patch also changes NetworkStatsAccessTest to use up-to-date testing
style and not depend on the deprecated junit.framework.TestCase.

Bug: 32561414
Test: runtest -x frameworks/base/../NetworkStatsAccessTest.java
Change-Id: Ib78f137578cf35e1c766b377b7f812a09173c49e