NativeHelper: Do not allow arbitrary library strings in user builds

On device, only allow "libart.so" in non-debuggable (user) builds.

Bug: 16404669

(cherry picked from commit 5019faa3a6bdde33785ece0537e8b865ea7cf938)

Change-Id: Ie163c04ce40c82698dcc98ced651dafef094d8b5
diff --git a/include/nativehelper/JniInvocation.h b/include/nativehelper/JniInvocation.h
index 9876d8d..b5198ff 100644
--- a/include/nativehelper/JniInvocation.h
+++ b/include/nativehelper/JniInvocation.h
@@ -38,6 +38,9 @@
   // persist.sys.dalvik.vm.lib.
   bool Init(const char* library);
 
+  // Exposes which library is actually loaded from the given name.
+  static const char* GetLibrary(const char* library);
+
  private:
 
   bool FindSymbol(void** pointer, const char* symbol);