Adding Manage External Sources Settings

Added a settings fragment to manage external sources. It lists all
applications that have either requested REQUEST_INSTALL_PACKAGES or have
their app op changed from default.

Test: Will include in follow-up CL. Tracked in b/33792674

Bug: 31002700
Change-Id: Ibd2a1922be214b62aec4eefa45f7b9691256b205

src/com/android/settings/Settings.java

@@ -138,6 +138,9 @@ public class Settings extends SettingsActivity {
     public static class AppWriteSettingsActivity extends SettingsActivity { /* empty */ }
     public static class AdvancedAppsActivity extends SettingsActivity { /* empty */ }
 
+    public static class ManageExternalSourcesActivity extends SettingsActivity {
+        /* empty */ }
+
     public static class WifiCallingSuggestionActivity extends SettingsActivity { /* empty */ }
     public static class ZenModeAutomationSuggestionActivity extends SettingsActivity { /* empty */ }
     public static class FingerprintSuggestionActivity extends FingerprintSettings { /* empty */ }