Add a null home activity to Shell.apk

In configurations where AOSP Settings.apk is not included, there might
not be any home activity. E.g. CSI doesn't have Settings.apk.

This CL adds a null home activity to Shell.apk, which is in CSI,
so the framework can be in a sane state. Without such null home
activity in CSI, package manager fails to install any APKs.

Bug: 148044842
Test: $ lunch csi_arm64-userdebug; m -j
      # Flash CSI to a Pixel (with AVB disabled) and reboot
      $ m DeviceHealthChecks
      $ adb install $OUT/testcases/DeviceHealthTests/arm64/DeviceHealthTests.apk
      Performing Streamed Install
      Success

Change-Id: I87999e30298bbe42af64f28e88fc6dc5975f1ca3