fix: consistent names for all cdk directives

[jelbourn]

Based on #7544. Should be fully backwards compatible.

Renames:
OverlayOrigin -> CdkOverlayOrigin
ConnectedOverlayDirective -> CdkConnectedOverlay
PortalDirective -> CdkPortal
PortalHostDirective -> CdkPortalOutlet
ObserveContent -> CdkObserveContent

Also stop using "mat" completely in the cdk.

BREAKING CHANGE:
MAT_CONNECTED_OVERLAY_SCROLL_STRATEGY is renamed to
CDK_CONNECTED_OVERLAY_SCROLL_STRATEGY

[crisbeto]

LGTM

[crisbeto]

I was thinking about this one a few days ago: do we really need the directive? It doesn't really provide a lot of value since using it ends up being slightly longer than straight-up passing in the elementRef to the overlay:

<button cdk-overlay-origin #origin="cdkOverlayOrigin">Open</button>
<ng-template cdk-connected-overlay [origin]="origin"></ng-template>

Versus something like this if we got rid of it:

<button #origin>Open</button>
<ng-template cdk-connected-overlay [origin]="origin"></ng-template>

It's not something we should necessarily address now, but it just something to think about.

[jelbourn]

The problem is that if the element you're applying it do has a component (like mat-button), you don't have a way to grab the ElementRef in the template.

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