Fix a NPE when clicking encryption interstitial header

Pref tree handler cannot assume preference has non-null key.

Change-Id: I790e75f3b4c888e2a48b297bea89cda0b8dbaabd
Fix: 35855587
Test: runtest --path tests/app/src/com/android/settings/EncryptionInterstitialTest.java
3 files changed