Don't crash if we get a onRemove() callback from Fingerprint

Before this fix, it was possible to get a callback after the activity
finishes.  This just ensures the operation is a no-op.

Fixes bug 37106041

Test: remove all but last fingerprint and hit back while rotating; check that
Settings doesn't crash.

Change-Id: I99df4f7c06e345d1ed6041a75606b0cc43edb300

src/com/android/settings/fingerprint/FingerprintSettings.java

@@ -360,6 +360,8 @@ public class FingerprintSettings extends SubSettings {
         }
 
         private void updateAddPreference() {
+            if (getActivity() == null) return; // Activity went away
+
             /* Disable preference if too many fingerprints added */
             final int max = getContext().getResources().getInteger(
                     com.android.internal.R.integer.config_fingerprintMaxTemplatesPerUser);