Implement addBigramWords() for DynamicPatriciaTrie.

...licyimpl/dictionary/bigram/bigram_list_read_write_utils.h

...licyimpl/dictionary/bigram/dynamic_bigram_list_policy.cpp

...policyimpl/dictionary/bigram/dynamic_bigram_list_policy.h

...licyimpl/dictionary/dynamic_patricia_trie_node_reader.cpp

...yimpl/dictionary/dynamic_patricia_trie_writing_helper.cpp

...icyimpl/dictionary/dynamic_patricia_trie_writing_helper.h

.../com/android/inputmethod/latin/BinaryDictionaryTests.java