Use default DSU slot name when active DSU is empty

If host system is Android Q and guest system is S, then reenable DSU in
the guest system (`adb shell gsi_tool enable -s`) would create an empty
DSU active indicator file.
In this case, use the default slot name "DSU" if DSU active indicator
file is non-existent or empty.

Bug: 172541102
Bug: 168571434
Test: Boot GSI S on Pixel Q with DSU
Test: Verify reenable DSU works
Test: adb shell gsi_tool enable -s && adb reboot
Change-Id: Ie59e4378d1aa4dca085c399a75abd4e662340030