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