Change types of fields of network stats reported to framework.

Currently, NetworkStats use int to handle uid, set and tag,
while native side using unsigned int mixing with signed int
with that. This commit make necessary changes in JNI part while
libnetdbpf unifying the types of fields.

Bug: 112226716
Test: 1. manually reconnect vpn
      2. update apps from playstore
      3. atest libnetdbpf_test
      4. runtest frameworks-net
      5. cts-tradefed run cts -m CtsUsageStatsTestCases -t \
              android.app.usage.cts.NetworkUsageStatsTest

Change-Id: I6c27124db8292e2825fba51b8994f013897cb566