Add initial multi-display support.

Split the DisplayManager into two parts.  One part is bound
to a Context and takes care of Display compatibility and
caching Display objects on behalf of the Context.  The other
part is global and takes care of communicating with the
DisplayManagerService, handling callbacks, and caching
DisplayInfo objects on behalf of the process.

Implemented support for enumerating Displays and getting
callbacks when displays are added, removed or changed.

Elaborated the roles of DisplayManagerService, DisplayAdapter,
and DisplayDevice.  We now support having multiple display
adapters registered, each of which can register multiple display
devices and configure them dynamically.

Added an OverlayDisplayAdapter which is used to simulate
secondary displays by means of overlay windows.  Different
configurations of overlays can be selected using a new
setting in the Developer Settings panel.  The overlays can
be repositioned and resized by the user for convenience.

At the moment, all displays are mirrors of display 0 and
no display transformations are applied.  This will be improved
in future patches.

Refactored the way that the window manager creates its threads.
The OverlayDisplayAdapter needs to be able to use hardware
acceleration so it must share the same UI thread as the Keyguard
and window manager policy.  We now handle this explicitly as
part of starting up the system server.  This puts us in a
better position to consider how we might want to share (or not
share) Loopers among components.

Overlay displays are disabled when in safe mode or in only-core
mode to reduce the number of dependencies started in these modes.

Change-Id: Ic2a661d5448dde01b095ab150697cb6791d69bb5