Shell: Move DisplayManager initialization to a separate method

This CL:
* Moves a bunch of related DisplayManager and DisplayConfigurator
  code to a separate InitializeDisplayManager method to help improve
  managability of Shell::Init.
* Renames ShutdownObserver -> DisplayShutdownObserver.
* Removes unnecessary DBusThreadManager dependencies from shell.cc.
* Moves observer responsibility from Shell to ProjectingObserver

Bug: 678949
Change-Id: I07159d0562bc032f03c653beb927ab8aaf053bbc
Reviewed-on: https://chromium-review.googlesource.com/833500
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: James Cook <[email protected]>
Commit-Queue: Steven Bennetts <[email protected]>
Cr-Commit-Position: refs/heads/master@{#526741}
8 files changed