Make Vpn.isAvailable return available, not unavailable

Also removed the check about secondary users. We already disable
adding a legacy VPN for secondary users inside the Settings page
itself.

We do not want to stop our users from configuring/disabling third
party VPNs, that's completely supported.

Bug: 23693383
Fix: 32308780
Test: manual, needs robotest
Change-Id: I99ab14b6b26582bdd7fc491c4b2542f3e653f3dc

src/com/android/settings/network/VpnPreferenceController.java

@@ -59,8 +59,7 @@ public class VpnPreferenceController extends PreferenceController {
 
     @Override
     public boolean isAvailable() {
-        // TODO: http://b/23693383
-        return mIsSecondaryUser || RestrictedLockUtils.hasBaseUserRestriction(mContext,
+        return !RestrictedLockUtils.hasBaseUserRestriction(mContext,
                 UserManager.DISALLOW_CONFIG_VPN, UserHandle.myUserId());
     }