Add isAvailable check to devOptionsController

- This avoids the null check for PreferenceControllers where isAvailable()
 is not always true

Bug: 34203528
Test: make RunSettingsRoboTests -j40
Change-Id: Ibed8bc6a2a812355c521620d77fb571c1fd8a649

src/com/android/settings/development/OemUnlockPreferenceController.java

@@ -122,12 +122,12 @@ public class OemUnlockPreferenceController extends DeveloperOptionsPreferenceCon
     }
 
     @Override
-    public void onDeveloperOptionsEnabled() {
+    protected void onDeveloperOptionsSwitchEnabled() {
         handleDeveloperOptionsToggled();
     }
 
     @Override
-    public void onDeveloperOptionsDisabled() {
+    protected void onDeveloperOptionsSwitchDisabled() {
         handleDeveloperOptionsToggled();
     }
 
@@ -143,10 +143,6 @@ public class OemUnlockPreferenceController extends DeveloperOptionsPreferenceCon
     }
 
     private void handleDeveloperOptionsToggled() {
-        if (mPreference == null) {
-            return;
-        }
-
         mPreference.setEnabled(enableOemUnlockPreference());
         if (mPreference.isEnabled()) {
             // Check restriction, disable mEnableOemUnlock and apply policy transparency.