ueventd: Break devices.cpp into discrete classes

devices.cpp handles too many things for creating one class.  This
change breaks it up into various files and classes.

* Parsing is moved to ueventd_parser.cpp
* Reading from the uevent socket and Cold booting is moved to a
  UeventListener class, in uevent_listener.cpp
* Firmware handling is moved to firmware_handler.cpp
* The remaining contents form a DeviceHandler class within devices.cpp

Bug: 33785894

Test: boot bullhead x40, observe no major differences in /dev and /sys
Test: boot sailfish x40, observe no major differences in /dev and /sys
Test: init unit tests

Change-Id: I846a2e5995fbb344c7a8e349065c18a934fa6aba