fix(drawer): not restoring focus on close

Currently the drawer restores focus on close after its closing animation is done and it contains the currently-focused element. This will always evaluate to false, because the sidenav gets a `visibility: hidden` which will blur any focused elements that it may have. With these changes the drawer will restore focus when the animation starts and it's still visible.

Author: crisbeto

src/lib/sidenav/drawer.ts

@@ -212,8 +212,6 @@ export class MatDrawer implements AfterContentInit, OnDestroy {
         this._focusTrap.focusInitialElementWhenReady();
       }
     });
-
-    this.onClose.subscribe(() => this._restoreFocus());
   }
 
   /**
@@ -302,7 +300,13 @@ export class MatDrawer implements AfterContentInit, OnDestroy {
   }
 
   _onAnimationStart(event: AnimationEvent) {
+    const {fromState, toState} = event;
+
     this._animationStarted.emit(event);
+
+    if (fromState !== toState && toState === 'void') {
+      this._restoreFocus();
+    }
   }
 
   _onAnimationEnd(event: AnimationEvent) {