In MobileNetworkSettings, add divider and "Advanced" button.

Setting style of PreferenceCategory to be the one with divider
above it. Manually add divider above APN preference. Added
expandable "Advanced" button to hide most preferences by default.

Bug: 63133738
Test: Manual

Change-Id: Ied9c025ab7288505262ba0c0222246ee74b30ef2
diff --git a/res/values/strings.xml b/res/values/strings.xml
index 0957b02..bc8dae1 100644
--- a/res/values/strings.xml
+++ b/res/values/strings.xml
@@ -489,6 +489,8 @@
     <string name="data_usage_title">Data usage</string>
     <!-- Summary about how much data has been used in a date range [CHAR LIMIT=100] -->
     <string name="data_usage_template"><xliff:g name="amount" example="200 MB">%1$s</xliff:g> mobile data used between <xliff:g name="date_range" example="Jan 1 -- Feb 2">%2$s</xliff:g></string>
+    <!-- Mobile network settings screen, Advanced button to show more items when clicked [CHAR LIMIT=50] -->
+    <string name="advanced_options_title">Advanced</string>
     <!-- Mobile network settings screen, name of the option to manage carrier profiles on devices which support embedded carrier profiles -->
     <string name="carrier_settings_euicc">Carrier</string>
     <!-- Mobile network settings screen, summary of the option to manage carrier profiles on devices which support embedded carrier profiles -->