fix(schematics): avoid lint warning in code generated by nav schematic (#16088)

Fixes a lint warning in the code that is generated by the `nav` schematic. The lint rule that fails is the `template-no-negated-async` from Codelyzer.

Fixes #16085.

Author: crisbeto

src/material/schematics/ng-generate/nav/files/__path__/__name@dasherize@if-flat__/__name@dasherize__.component.html

@@ -1,8 +1,8 @@
 <mat-sidenav-container class="sidenav-container">
-  <mat-sidenav #drawer class="sidenav" fixedInViewport="true"
+  <mat-sidenav #drawer class="sidenav" fixedInViewport
       [attr.role]="(isHandset$ | async) ? 'dialog' : 'navigation'"
       [mode]="(isHandset$ | async) ? 'over' : 'side'"
-      [opened]="!(isHandset$ | async)">
+      [opened]="(isHandset$ | async) === false">
     <mat-toolbar>Menu</mat-toolbar>
     <mat-nav-list>
       <a mat-list-item href="#">Link 1</a>