Add a method to iterate ngram entries.

native/jni/src/suggest/core/dictionary/dictionary.cpp

native/jni/src/suggest/core/dictionary/dictionary.h

native/jni/src/suggest/core/dictionary/ngram_listener.h

...est/core/policy/dictionary_structure_with_buffer_policy.h

...ary/structure/backward/v402/ver4_patricia_trie_policy.cpp

...onary/structure/backward/v402/ver4_patricia_trie_policy.h

...licyimpl/dictionary/structure/v2/patricia_trie_policy.cpp

...policyimpl/dictionary/structure/v2/patricia_trie_policy.h

...mpl/dictionary/structure/v4/ver4_patricia_trie_policy.cpp

...yimpl/dictionary/structure/v4/ver4_patricia_trie_policy.h