Extend "headless system" app-standby exemption

Originally we only exempted system apps with zero activities from app-standby.

Now we exempt system apps with no "front door" activities -- i.e. system apps
with no launcher icons.

The diff is available here: https://diff.googleplex.com/#key=JUzxUtk7VRJy

Bug: 222111596
Bug: 186261675
Test: atest AppStandbyControllerTests
Test: manual inspection of `dumpsys usagestats  | sed -n '/^mHeadlessSystemApps/,/^\]/p'`
Test: Run `adb install -r -g $OUT/product/priv-app/PixelLiveWallpaperPrebuilt/PixelLiveWallpaperPrebuilt.apk`
and make sure `com.google.pixel.livewallpaper` is still in the above dumpsys list

Change-Id: I8cd5cbd5f5e6ec42668f64df02fa718c0ac915e4