Fix dependency between keyguard/dexopt/AccessibilityManagerService.

This fixes a bug on userdebug builds where dexopt hadn't completed
before the service is needed.  The resulting crash prevented dexopt
from completing, resulting in a reboot loop and perpetual issue starting
the service.

The solution is to hold off on creating keyguard until the system is ready.

It also moves the creation of the AccessibilityManagerService singleton back
into the system process.

Fixes bug 8287106

Change-Id: I96e15c084d38c255fd8f6bca6f21a0785b82ec74