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
(cherry picked from commit d5a3c5373315e1eefe0f0312aa19007313461634)
diff --git a/src/com/android/settings/fingerprint/FingerprintSettings.java b/src/com/android/settings/fingerprint/FingerprintSettings.java
index 0f48f8c..dc6616a 100644
--- a/src/com/android/settings/fingerprint/FingerprintSettings.java
+++ b/src/com/android/settings/fingerprint/FingerprintSettings.java
@@ -360,6 +360,8 @@
         }
 
         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);