Add ability to show/hide assist_and_voice_input preference.

This adds a new boolean flag, config_show_assist_and_voice_input, which
when set to false will hide the assist_and_voice_input preference item.

Bug: 62379282
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=DefaultAssistPreferenceControllerTest
Change-Id: I3bfc84f0fba07cd52679269daadbdcfccd205ff1