Add the new format of bigram entries.

In new format, each bigram entry has flags (1 byte), a terminal id (3 byte),
a time-stamp (4 byte), a counter (1 byte) and a level (1 byte).

Bug: 10920255
Bug: 10920165
Change-Id: I0f7fc125a6178e6d25a07e8462afc41a7f57e3e1