[Physical Keybard] add Mouse key main page

Add page for Mouse key, which contain a main switch toggle and list of
explain images.
demo video: b/346949547#comment38
images:
https://screenshot.googleplex.com/3mc6KnyMdvfAia9.png
https://screenshot.googleplex.com/489mTfSYg9KMUpW.png

Bug: 346949547
Test: atest SettingsRoboTests
Flag: com.android.settings.keyboard.keyboard_and_touchpad_a11y_new_page_enabled
Change-Id: Ia40d5f071cc674ce0118d7ec8a4f0d5e914ce8b9