Add tests to Launcher P0 CUJ Homescreen navigation via TAPL.

1. Implement API to help to get page count.
2. Add test add and delete page.

Bug: 241685418
Bug: 232584721
Test: android.platform.test.scenario.launcher.integration_tests.ScreenNavigationTests#testAddAndDeletePageAndFling
Change-Id: I2e04b9ef1f2ca65ba14d6939b7b51fead6b6f6ca

tests/src/com/android/launcher3/ui/TaplTestsLauncher3.java

@@ -214,9 +214,9 @@ public class TaplTestsLauncher3 extends AbstractLauncherUiTest {
         // Test that ensureWorkspaceIsScrollable adds a page by dragging an icon there.
         executeOnLauncher(launcher -> assertFalse("Initial workspace state is scrollable",
                 isWorkspaceScrollable(launcher)));
-        workspace.verifyWorkspaceAppIconIsGone(
-                "Chrome app was found on empty workspace", "Chrome");
-
+        assertEquals("Initial workspace doesn't have the correct page", workspace.pagesPerScreen(),
+                workspace.getPageCount());
+        workspace.verifyWorkspaceAppIconIsGone("Chrome app was found on empty workspace", "Chrome");
         workspace.ensureWorkspaceIsScrollable();
 
         executeOnLauncher(

tests/tapl/com/android/launcher3/tapl/Workspace.java

@@ -192,6 +192,12 @@ public final class Workspace extends Home {
         }
     }
 
+    /** Returns the number of pages. */
+    public int getPageCount() {
+        final UiObject2 workspace = verifyActiveContainer();
+        return workspace.getChildCount();
+    }
+
     /**
      * Returns the number of pages that are visible on the screen simultaneously.
      */