Refactor display manager service to new pattern.

Transform DisplayManagerService into a SystemService and start cleaning
up other local services that it uses from window manager and input manager.

Clean up service thread initialization.

Remove unnecessary static variables from ActivityManagerService.

It's starting to become clear that we really need a better way to manage
service dependencies.  Boot phases don't quite cut it.

Change-Id: If319dbd7cbfbd4812fe55ece969e818d4b20755b