Add primary key switch preference in mousekeys page

This change only adds the UI. Will implement functionalities in
a follow up change once the usePrimaryKeys is added in settings.

Screenshot: go/screenshot-7vgv58wfps4tid7

Bug: 393569995
Test: atest MouseKeysPrimaryKeysControllerTest
Flag: com.android.server.accessibility.enable_mouse_key_enhancement
Change-Id: I789d87a43ccff9002d5cc37f28127675498fd8ec