Merge "Support bigram historical information migration."

...ve/jni/com_android_inputmethod_latin_BinaryDictionary.cpp

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

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

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

.../policyimpl/dictionary/bigram/ver4_bigram_list_policy.cpp

...st/policyimpl/dictionary/bigram/ver4_bigram_list_policy.h

...ionary/structure/pt_common/dynamic_pt_updating_helper.cpp

...ctionary/structure/pt_common/dynamic_pt_updating_helper.h

...olicyimpl/dictionary/structure/pt_common/pt_node_writer.h

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

...ictionary/structure/v4/ver4_patricia_trie_node_writer.cpp

.../dictionary/structure/v4/ver4_patricia_trie_node_writer.h

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

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

...roid/inputmethod/latin/BinaryDictionaryDecayingTests.java