liblog: add __android_log_pmsg_file_read

- This is considered an Android Private function, not exported
  for general use.
- goal is to retreive a file's content from a series of log
  messages from pmsg, to be retrieved after a reboot for
  transfer to a persistent location.
- files are presented in reverse sorted order, first based on
  _any_ numerical content, then by alphanumeric order.
- Add a gTest for this function, relies on gTest for
  liblog.__android_log_pmsg_file_write from prior to reboot.

Bug: 27176738
Change-Id: If37ef423009bd28b598b233af3bccef3429bdc22