Protect entry points to add an eSIM when admin restricted

If the DISALLOW_CONFIG_MOBILE_NETWORKS admin policy is set, we were
accidentally still allowing access to the flow where you add an eSIM
subscription via the "plus" button on the Network & internet page. While
fixing this, I also noticed that the mobile networks list page (which
only becomes available if you have multiple subscriptions) has a link at
the bottom to start the flow as well, and that wasn't being protected.

The fix for the plus button on the Network & internet page was just to
make sure not to call setEnabled(true) if the preference was already
disabled by admin policy, since that has the effect of overriding the
admin-disabling.

The fix for the mobile networks list page just needed to add the
relevant tags in the layout XML, and then we get it for free.

Fixes: 137627845
Test: make RunSettingsRoboTests
Change-Id: I896ac248f50aaeecc157791938a0a0a98265aa07

res/xml/mobile_network_list.xml

@@ -20,9 +20,11 @@
     android:key="mobile_network_list_screen"
     android:title="@string/network_settings_title">
 
-    <Preference
+    <com.android.settingslib.RestrictedPreference
         android:key="add_more"
         settings:isPreferenceVisible="false"
+        settings:userRestriction="no_config_mobile_networks"
+        settings:useAdminDisabledSummary="true"
         android:title="@string/mobile_network_list_add_more"
         android:icon="@drawable/ic_menu_add_activated_tint"
         android:order="100" >
@@ -30,6 +32,6 @@
             <extra android:name="android.telephony.euicc.extra.FORCE_PROVISION"
                    android:value="true"/>
         </intent>
-    </Preference>
+    </com.android.settingslib.RestrictedPreference>
 
 </PreferenceScreen>