Move socket tagging implementation to mainline.

Currently, socket tagging is implemented in the framework. The
Java code is in NetworkManagmentSocketTagger.java and the JNI is
in libandroid and loaded by the general framework JNI loader.

- Move the Java implementation to TrafficStats, which is the only
  caller of NetworkManagmentSocketTagger. This simplifies the
  code a bit because a number of methods can be deleted.

- Move the JNI code to a new JNI library in the APEX. The library
  depends only on the NDK and statically links the NDK-only
  version of libnativehelper. Its size is only 5k on ARM and 10k
  on ARM64.

- Temporarily make the framework depend on this library until the
  rest of the T connectivity code moves to the APEX.

Test: atest NetworkUsageStatsTest CtsNetTestCases:TrafficStatsTest
Change-Id: I050c7c515237f68b78d08987bc443f50a7949c06