Add tagSocket/untagSocket NDK APIs

Provide native APIs to tag the various sockets used for network
transfer, which helps apps tracking down data usage inside their app.

We've had equivalent Java APIs since API level 14.
https://developer.android.com/reference/android/net/TrafficStats.html#tagSocket(java.net.Socket)
https://developer.android.com/reference/android/net/TrafficStats.html#untagSocket(java.net.Socket)

Given that these Java APIs are implemented in native side, providing
NDK APIs directly will be easier for native users to use it.

Bug: 29761778
Bug: 202086915
Test: Tests in change Ic6554d9b98cef9b41b7eae06315d284c574c0a45
Change-Id: I802ce62f82199ce087bf8feb03afb26de1ce526e