Use NewOject to invoke constructor of NetworkStats.Entry

AllocObject doesn't invoke the constructor, and when working on
Java code we wouldn't expect that stuff initialized in the
constructor wouldn't actually be initialized on instances
created from JNI.

Test: atest NetworkStackIntegrationTests:android.net.NetworkStatsIntegrationTest
Fix: 322456911
Change-Id: Ibc87ab5ebaf02a7bcaa88307511bd3aaea8698ed