AssetManager: Guard against null in array retrieval

Resources does the right thing and throws NotFoundException
if the result from AssetManager.getResourceTextArray() is null,
but AssetManager itself doesn't guard from the native implementation
returning null.

This lets callers always protect against missing resources via
catching NotFoundException.

Change-Id: I4f7ad32819bc9598efb28dbb10a59ff5201ecfca
Test: manual