Restrict DisplayContent creation.

This changelist limits DisplayContent the display
container/controller creation. All other callpoints now can only
retrieve existing DisplayContents. This removes the chances of
arbitrary calls generating a DisplayContent without the related
ActivityDisplay on the ActivityManager side.

Since display creation is now driven on the ActivityManager side,
the ActivityManagerService must be initialized and associated with
the WindowManagerService before the WindowManagerService can be fully
initialized. This is reflected in changes to SystemServer.

Fixes: 72228411
Test: go/wm-smoke-auto
Change-Id: I8dddb5fc109be4363de5ba87faff9d9885009042