fix(material/snack-bar): change politeness level (#30851)

* fix(material/snack-bar): change politeness level

change politeness level of snack bar to polite to match docs

fixes b/364250804

* fix(material/snack-bar): update unit tests

update unit tests

fixes b/364250804

Author: DBowen33

src/material/snack-bar/snack-bar-config.ts

@@ -24,7 +24,7 @@ export type MatSnackBarVerticalPosition = 'top' | 'bottom';
  */
 export class MatSnackBarConfig<D = any> {
   /** The politeness level for the MatAriaLiveAnnouncer announcement. */
-  politeness?: AriaLivePoliteness = 'assertive';
+  politeness?: AriaLivePoliteness = 'polite';
 
   /**
    * Message to be announced by the LiveAnnouncer. When opening a snackbar without a custom

src/material/snack-bar/testing/snack-bar-harness.spec.ts

@@ -76,11 +76,11 @@ describe('MatSnackBarHarness', () => {
   it('should be able to get aria-live of snack-bar', async () => {
     fixture.componentInstance.openCustom();
     let snackBar = await loader.getHarness(MatSnackBarHarness);
-    expect(await snackBar.getAriaLive()).toBe('assertive');
+    expect(await snackBar.getAriaLive()).toBe('polite');
 
-    fixture.componentInstance.openCustom({politeness: 'polite'});
+    fixture.componentInstance.openCustom({politeness: 'assertive'});
     snackBar = await loader.getHarness(MatSnackBarHarness);
-    expect(await snackBar.getAriaLive()).toBe('polite');
+    expect(await snackBar.getAriaLive()).toBe('assertive');
 
     fixture.componentInstance.openCustom({politeness: 'off'});
     snackBar = await loader.getHarness(MatSnackBarHarness);