DSU enhancement (3/N)

* Expose KEY_ONE_SHOT
  This allows the device to stay in DynamicSystem mode
  across reboots until the user decides to exit DSU mode

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 \
  --ez KEY_ONE_SHOT false
Change-Id: I8057ebb7af866f9da3ad7836615866642ff93ddb