Add backdrop to autocomplete overlay

[msgoloborodov]

mat-select has ovarlay with transparent backdrop, but mat-autocomplete hasn't.
It almost same components, and it should has same behavior.

_getOverlayConfig method of MatAutocompleteTrigger should also return { hasBackdrop: true, backdropClass: 'cdk-overlay-transparent-backdrop' }.

components/src/material/autocomplete/autocomplete-trigger.ts

Line 903 in b088604

panelClass: this._defaults?.overlayPanelClass,

[crisbeto]

Duplicate of #30457.

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