Updated JNI interface.

Added new "ping" method used to detect if the shared
library has already been loaded.

Fixed a few bugs in the implementation (and now it works
from the Java side).

See http://ag/1590668 for the implementation on devtools.

Test: Done on the tools side
Change-Id: Ifa41073dc3b6ac69cdb9f77e655bf261c3b4c3d1