DSU enhancement (2/N)

* Expose KEY_ENABLE_WHEN_COMPLETED
  This will streamline the UI flow where
  the device will reboot immediately when the installation is
  done.

Bug: 277691885
Test: adb shell am start-activity \
  -n com.android.dynsystem/com.android.dynsystem.VerificationActivity \
  -a android.os.image.action.START_INSTALL \
  --el KEY_USERDATA_SIZE 2147483648 \
  --ez KEY_ENABLE_WHEN_COMPLETED true
Change-Id: I63cfd66647ac0cfc312ef329d18aba89b93aa990