Start untangling system server early bootstrapping.

Refactored SystemServer to get rid of a bunch of legacy cruft related
to how the ServerThread used to be started up.

Create system context first when system server starts.  This removes
the tangled initialization order dependency that forced us to start
the activity manager service before most anything else.

Moved factory test related constants into the FactoryTest class.

Partially migrated Installer, ActivityManagerService, and
PowerManagerService to the new SystemService pattern.  There's more
work to be done here, particularly around the lifecycle of the
power manager.

Bug: 12172368
Change-Id: Ia527dd56e3b3fd90f9eeb41289dbe044921230d4