Add byte size of the bytes fields to statslog c++ APIs.

The proto binary data can contain '\0's and in the native layer,
the current liblog api would convert that into string and thus
the data is truncated.

This CL adds a "size_t bytes_field_len" after the bytes fields so that
we can correctly pass the data from JAVA to native.

Java StatsLog.write() APIs remain the same

Test: test_drive with atom 103
Change-Id: I34f1c4ddd6a4ec5f3604b0c67a47a5399e3c6ddd