[Physical Keyboard] update color of Dialog

1. update color of slider in dialog to increase contrast
2. not showing device name in Mouse key page if there's only one device
   connected
Bug: 374030364
Bug: 377464791
Flag: com.android.settings.keyboard.keyboard_and_touchpad_a11y_new_page_enabled
Test: atest
packages/apps/Settings/tests/robotests/src/com/android/settings/inputmethod/

Change-Id: I94d7859ba1dfdad7fdca2ee750a5afeb881756f4