Fix search indexing for encryption_and_credential page

- Rewrite search index provider to include all possible keys through
  getXmlResourcesToIndex()
- Add isPageSearchEnabled() to disable all keys if user is not admin
- Add getNonIndexableKeys to suppress unrelated keys based on current
  device state

Change-Id: I2c6943483789bf4c9f3931d344cf279fec0edaee
Fix: 37650170
Test: robotests

res/xml/security_settings_unencrypted.xml

@@ -18,14 +18,14 @@
                   android:title="@string/crypt_keeper_settings_title">
 
     <PreferenceCategory
-        android:key="security_category"
+        android:key="security_category_for_unencrypted_device"
         android:title="@string/crypt_keeper_settings_title">
 
         <Preference
             android:key="encryption"
             android:title="@string/crypt_keeper_encrypt_title"
             android:fragment="com.android.settings.CryptKeeperSettings" />
-    
+
     </PreferenceCategory>
 
 </PreferenceScreen>