Return Entry from TrafficStats JNI

This is no-op refactoring which returns [tx|rx][Bytes|Packets]
to the service instead of passing type to JNI and return only
1 value.
This is needed for the follow-up changes to caching values in
a row in the service side.

This change also remove unused JNI methods.

Test: atest NetworkStackIntegrationTests:android.net.NetworkStatsIntegrationTest
Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.NetworkStatsServiceTest
Bug: N/A
Change-Id: I3b6ac97e6a1bff49cffe6153ffc2618f37a9deb4