fix(material/form-field): ensure that focused classes are in sync (#31568)

This is something that showed up in some internal tests a while ago. Because we set `mat-focused` through a host binding while `mdc-text-field--focused` is set through direct DOM manipulation, nothing guarantees that they'll be in sync and in some internal tests they aren't.

These changes sync both of them up from the same place.

Author: crisbeto

src/material/form-field/form-field.ts

@@ -160,7 +160,6 @@ interface MatFormFieldControl<T> extends _MatFormFieldControl<T> {}
     '[class.mat-form-field-appearance-fill]': 'appearance == "fill"',
     '[class.mat-form-field-appearance-outline]': 'appearance == "outline"',
     '[class.mat-form-field-hide-placeholder]': '_hasFloatingLabel() && !_shouldLabelFloat()',
-    '[class.mat-focused]': '_control.focused',
     '[class.mat-primary]': 'color !== "accent" && color !== "warn"',
     '[class.mat-accent]': 'color === "accent"',
     '[class.mat-warn]': 'color === "warn"',
@@ -340,6 +339,7 @@ export class MatFormField
   private _stateChanges: Subscription | undefined;
   private _valueChanges: Subscription | undefined;
   private _describedByChanges: Subscription | undefined;
+  private _outlineLabelOffsetResizeObserver: ResizeObserver | null = null;
   protected readonly _animationsDisabled = _animationsDisabled();
 
   constructor(...args: unknown[]);
@@ -544,27 +544,25 @@ export class MatFormField
   }
 
   private _updateFocusState() {
+    const controlFocused = this._control.focused;
+
     // Usually the MDC foundation would call "activateFocus" and "deactivateFocus" whenever
     // certain DOM events are emitted. This is not possible in our implementation of the
     // form field because we support abstract form field controls which are not necessarily
     // of type input, nor do we have a reference to a native form field control element. Instead
     // we handle the focus by checking if the abstract form field control focused state changes.
-    if (this._control.focused && !this._isFocused) {
+    if (controlFocused && !this._isFocused) {
       this._isFocused = true;
       this._lineRipple?.activate();
-    } else if (!this._control.focused && (this._isFocused || this._isFocused === null)) {
+    } else if (!controlFocused && (this._isFocused || this._isFocused === null)) {
       this._isFocused = false;
       this._lineRipple?.deactivate();
     }
 
-    this._textField?.nativeElement.classList.toggle(
-      'mdc-text-field--focused',
-      this._control.focused,
-    );
+    this._elementRef.nativeElement.classList.toggle('mat-focused', controlFocused);
+    this._textField?.nativeElement.classList.toggle('mdc-text-field--focused', controlFocused);
   }
 
-  private _outlineLabelOffsetResizeObserver: ResizeObserver | null = null;
-
   /**
    * The floating label in the docked state needs to account for prefixes. The horizontal offset
    * is calculated whenever the appearance changes to `outline`, the prefixes change, or when the