Refine the exported DSU status

The new exported DSU status removes the need to make blocking binder
calls out of system server during device boot.

Bug: 149790245
Bug: 149716497
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: I27fae316214498407a73474ca8b93aec3518e4b5