liblog: add __android_log_pmsg_file_write

- This is considered an Android Private function, not exported
  for general use.
- goal is to record a file's content into a series of log
  messages into pmsg, to be retrieved after a reboot for
  transfer to a persistent location.
- filename reference is converted to a tag-unique
  "<dirbase>:<filebase>".
- buffer and length representing the filename contents are
  recorded, along with a sequence number placed into the nsec
  time field to ANDROID_LOG_PMSG_FILE_MAX_SEQUENCE.
- Add a gTest for this function.

Bug: 27176738
Change-Id: If93df3ae8bfc1bb75516d4a1fd8dae0301af644b