Restrict root/superuser install options to Experts

These two options are only enabled when Expert Mode is switched on.
They're far too incomplete and risky currently, unless you know what
you're doing.

src/org/fdroid/fdroid/views/fragments/PreferenceFragment.java

@@ -140,6 +140,16 @@ public class PreferenceFragment
         } else if (key.equals(Preferences.PREF_EXPERT)) {
             onoffSummary(key, R.string.expert_on,
                 R.string.expert_off);
+            CheckBoxPreference pref = (CheckBoxPreference) findPreference(key);
+            Preference rinst = findPreference(Preferences.PREF_ROOT_INSTALLER);
+            Preference sinst = findPreference(Preferences.PREF_SYSTEM_INSTALLER);
+            if (pref.isChecked()) {
+                rinst.setEnabled(true);
+                sinst.setEnabled(true);
+            } else {
+                rinst.setEnabled(false);
+                sinst.setEnabled(false);
+            }
 
         } else if (key.equals(Preferences.PREF_ROOT_INSTALLER)) {
             onoffSummary(key, R.string.root_installer_on,