Implement ver4 bigram reading method.

...id/inputmethod/latin/makedict/BinaryDictEncoderUtils.java

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

...mpl/dictionary/structure/v4/content/bigram_dict_content.h

...ctionary/structure/v4/content/sparse_table_dict_content.h

...ary/structure/v4/content/terminal_position_lookup_table.h

...st/policyimpl/dictionary/structure/v4/ver4_dict_buffers.h

...olicyimpl/dictionary/structure/v4/ver4_dict_constants.cpp

.../policyimpl/dictionary/structure/v4/ver4_dict_constants.h

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

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

.../src/suggest/policyimpl/dictionary/utils/sparse_table.cpp

.../android/inputmethod/latin/Ver4BinaryDictionaryTests.java