updater_sample: Build SystemUpdaterSample as non-privileged app by default.

Prior to the change, SystemUpdaterSample was built as privileged app.
It must be installed along with the matching privapp-permissions
whitelist change (as noted in updater_sample/README.md), otherwise would
block device booting if installed unintentionally (e.g. with `mm` or
`mmma`).

This CL avoids putting the device in a bad state due to a sample app.

Fixes: 135703777
Test: `lunch aosp_taimen-userdebug && m -j installclean &&
          mmma -j bootable/recovery && m -j`
      Flash on device and boot successfully.
Test: Run the tests per instructions in updater_sample/README.md.
Change-Id: Ib8b587c77570f05f7db748fad84744fa45016aab