Expose libstatspull as a stable C API

Libstatspull will ship as a part of the statsd apex in R.

Also update libstatssocket to have version 30 to align with api level,
as per native api council feedback.

Change both libraries to use NDK style naming conventions (AStatsEvent,
AStatsManager), and remove unneeded APIs in libstatssocket.

Remove KeyValuePairs from libstatssocket. KeyValuePairs will not be
supported in native code.

Bug: 147499386
Test: make libstatspull
Test: make libstatssocket
Test: atest libstatssocket_test
Change-Id: Ie79771461215a057529aaac91db95e4334c3960e