workloads/speedometer: Only close tabs on supported devices

Some devices don't have the option to close all tabs so don't error if
this element cannot be found.

wa/workloads/speedometer/uiauto/app/src/main/java/com/arm/wa/uiauto/speedometer/UiAutomation.java

@@ -110,6 +110,8 @@ public class UiAutomation extends BaseUiAutomation {
         menu.click();
         UiObject closetabs =
             mDevice.findObject(new UiSelector().textContains("Close all tabs"));
+        if (closetabs.exists()){
             closetabs.click();
         }
+    }
 }