Launch new Home app when selecting Home app in Settings

Launches the newly selected Home app when it has been selected in
Default Home app settings. This CL makes the change take effect
immediately, instead of when the Home button is pressed.

Bug: 70697830
Test: make RunSettingsRoboTests
Test: Manual - Change default home app in Settings
Change-Id: If7e2956bc6f8ddba0a7e8da48a0d38e3936c57e9