Workaround for apps using a copy of libnativehelper

When this library is loaded by app via greylist
it ends up having its own copy of the library which does
not have JNI constants properly initialized.

This workaround checks the initialization status and
initializes them lazily in this case.

Bug: http://b/27724707
Change-Id: I4d12913c86a7e8e86c910c90316fc0b94190ac94
(cherry picked from commit 3c916450)