DSU enhancement (1/N)

* System image is not required
  This allows the client to install a new data partition without
  supplying new system images. This is handy to simulate the
  condition of factory reset.

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
Change-Id: I0580b9b608221f4a07d7f8456de0cdd2f300578b