Finish setup activity when supervision lock is already set.

Context: this is to fix the issue that somehow the activity is brought to front and starts the pin recovery flow after unlocking device. Also updated the result code to be more random.

Change-Id: I164a25ddde508e8edea68aca091bc2d516694ca3
Bug: 416492063
Test: tests/robotests/src/com/android/settings/supervision/SetupSupervisionActivityTest
Flag: android.app.supervision.flags.enable_supervision_settings_screen