Adding dismiss-all method

See the bug.

Bug: 123259701
Change-Id: I4b41d2cfe14c53db1a99cb6a693d3047057fa925
Tests: testOverview

quickstep/tests/src/com/android/quickstep/TaplTestsQuickstep.java

@@ -176,6 +176,13 @@ public class TaplTestsQuickstep extends AbstractQuickStepTest {
         // Test UIDevice.pressHome, once we are in AllApps.
         mDevice.pressHome();
         waitForState("Launcher internal state didn't switch to Home", LauncherState.NORMAL);
+
+        // Test dismissing all tasks.
+        mLauncher.getWorkspace().switchToOverview().dismissAllTasks();
+        waitForState("Launcher internal state didn't switch to Home", LauncherState.NORMAL);
+        executeOnLauncher(
+                launcher -> assertEquals("Still have tasks after dismissing all",
+                        0, getTaskCount(launcher)));
     }
 
     private int getCurrentOverviewPage(Launcher launcher) {