Disable DSU in executeRebootToNormalCommand()

executeRebootToNormalCommand() reboots the device only if DSU is
successfully disabled, else show a toast describing the failure.

Bug: 171380622
Test: 1. Install and boot into a DSU
      2. adb shell gsi_tool enable
      3. Press "Restart" in DSU notification
      4. Observe the device reboots back to the original system
Change-Id: I19b3d09a64282a28ca7b99b441ca93dea45fcce0