As part of cleaning up JNI onload and initialization, this CL removes
the current code that builds up a vector of callbacks, which are
eventually run by base::android::OnJNIOnLoad{RegisterJNI,Init} in
favour of calling the appropriate code directly from those functions
(and the functions that call them).

BUG=678579
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2620303004
Cr-Commit-Position: refs/heads/master@{#448955}