Do not return synthetic launcher activity for completely headless apps

Bug: 140477182
Test: Headless app: no icon is shown on launcher
Test: App with launchable activity but disabled by default: no icon
Test: App with launchable activity, enabled by default but then disabled by command: has an icon on launcher
Change-Id: I58a36ac93d34583b2d4e50464b73cced77ce8edb