Fix enroll introduction page was not closed in background

Bug: 291023102
Test: 1. Launch FingerprintEnrollFindSensor and back to introducntion
      2. Go to home screen
      3. Launch FingerprintEnrollFindSensor from recent app
      4. Check if FingerprintEnrollFindSensor is stopped
      5. make RunSettingsRoboTests -j96 ROBOTEST_FILTER=com.android.settings.biometrics.fingerprint
Change-Id: I65504f663340a0e66d1f8bcd2e7d4b3659282884