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

src/com/android/settings/biometrics/BiometricEnrollIntroduction.java

@@ -236,6 +236,9 @@ public abstract class BiometricEnrollIntroduction extends BiometricEnrollBase
     protected void onResume() {
         super.onResume();
 
+        //reset mNextClick to make sure introduction page would be closed correctly
+        mNextClicked = false;
+
         final int errorMsg = checkMaxEnrolled();
         if (errorMsg == 0) {
             mErrorText.setText(null);