Modify android_dlwarning function to use a callback

The previous implementation of android_dlwarning was not thread-safe
and could return a pointer soon to become invalid in some situations.
This change fixed the problem. I have also removed android_dlwarning
from the dlext.h header file in case we decide to keep
android_dlwarning in the final release.

Bug: http://b/27453994
Change-Id: If6c896a80a17c4be0e18795e617712ad36a106fe