onLowMemory(), onTrimMemory() not called on Kindle Fire HD

Our applications are largely written in C++, I have hooked both the onLowMemory() and onTrimMemory() callback in the Java activity and forward the callback to our native code. This works fine on the older Kindle Fire and regular android devices, but neither of these callbacks seem to be triggered on the Fire HD (version 7.4.6). I have verified this by explicitly leaking memory and watching for the callbacks. Am I missing something? Should I be hooking some other callback to using some other method to query when memory is low?

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