Make KEY_DSU_SLOT work

OEMs can customize the software installation process to better
suit their specific needs.

Bug: 275484855
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 \
  --es KEY_DSU_SLOT test
Change-Id: I707976842eb1aefa1a3cb56532a44013f2422a95