Dismiss DSU notification in executeDiscardCommand()

Dismiss notification even if there is no active DSU installation.

Bug: 170072953
Test: Install a DSU system.
  Reboot into guest system and reboot back to host system.
  `adb shell gsi_tool wipe`
  Press "Discard" in the DSU notification, the notifiction is dismissed.
Change-Id: I6f217c4ba287e4789d61182a12485db60f9605c9