Don't add any trust agents if the controller is not available.

TrustAgentListPreferenceController gets a list of trust agents and add
them to the security_category preference category at runtime. However,
this update process is always ran - even if the controller is explicitly
disabled/not available. This fixes this.

Bug: None
Test: Updated TrustAgentListPreferenceControllerTest.
Change-Id: Ic95d219bc1d0c7cfa0c4eed0d3dd12dec0e14df6

src/com/android/settings/security/trustagent/TrustAgentListPreferenceController.java

@@ -146,6 +146,10 @@ public class TrustAgentListPreferenceController extends AbstractPreferenceContro
                 mSecurityCategory.removePreference(oldAgent);
             }
         }
+        // If for some reason the preference is no longer available, don't proceed to add.
+        if (!isAvailable()) {
+            return;
+        }
         // Then add new ones.
         final boolean hasSecurity = mLockPatternUtils.isSecure(MY_USER_ID);
         final List<TrustAgentManager.TrustAgentComponentInfo> agents =