[Physical Keyboard][A11y Page] Add new Keyboard A11y page

Add new Keyboard A11y page, and controled with flag.
Demo video: b/345399212#comment6

Bug: 345399212
Test: atest SettingsRoboTests
Flag: com.android.settings.keyboard.keyboard_and_touchpad_a11y_new_page_enabled
Change-Id: I4bd45ff16c8abb33e1cf56c377ea86c20195fe26