Add ability to show/hide default_home preference.

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

Bug: 62378256
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=DefaultHomePreferenceControllerTest

Change-Id: Ibbf17ebb76b61fc8b2eac84016e3d874ca15a513
5 files changed