Add support for android logcat text files

Wireshark already supports reading and writing logcat
logs saved in binary files. Binary format, although
better, is used less often than saving those logs to
text files.

This patch extends wireshark's support for android logcat
logs to reading and writing logcat logs in text files.

Features:
* support for tag, brief, process, thread, time, threadtime
  and long formats
* saving in original format
* it's generally awesome

Change-Id: I013d6ac2da876d9a2b39b740219eb398d03830f6
Reviewed-on: https://code.wireshark.org/review/1802
Reviewed-by: Anders Broman <a.broman58@gmail.com>