Support multiple DSU installations

Bug: 144247097
Test: adb shell am start-activity \
        -n com.android.dynsystem/com.android.dynsystem.VerificationActivity \
        -a android.os.image.action.START_INSTALL \
        -d file:///storage/emulated/0/Download/system.raw.gz \
        --el KEY_SYSTEM_SIZE $(du -b system.raw|cut -f1) \
        --el KEY_USERDATA_SIZE 8589934592
Change-Id: Ib2f86039c4c0c1dae2bc94a702721572567bcbf2