Enable System User Home for CSI

Core system image (CSI) doesn't have Launcher or Settings so no home
activity, but still needs to boot properly. This change enables the
fallback home activity, SystemUserHome, in the framework for CSI.

Bug: 149307007
Bug: 149780604
Test: $ lunch csi_arm64-userdebug; m
      # flash the system.img built to crosshatch
      $ atest DeviceHealthTests
Change-Id: I0f999f76e23c6316f6d5d0741a1116028e34a0ab