Tests for libstatspull

Sets up a new test suite in frameworks/base/apex/statsd. Creates a JNI
wrapper around libstatspull so that configs can be added and removed
from java, but the pulls happen in native code.

Bug: 145310729
Test: atest LibStatsPullTests
Change-Id: Ib3df74cac12f4276892be3f4a060a4a9dd120464