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

res/xml/special_access.xml

@@ -92,4 +92,15 @@
             android:name="classname"
             android:value="com.android.settings.Settings$UsageAccessSettingsActivity" />
     </Preference>
+
+    <Preference
+        android:key="manage_external_sources"
+        android:title="@string/install_other_apps"
+        android:fragment="com.android.settings.applications.ManageApplications"
+        settings:keywords="@string/keywords_install_other_apps">
+        <extra
+            android:name="classname"
+            android:value="com.android.settings.Settings$ManageExternalSourcesActivity" />
+    </Preference>
+
 </PreferenceScreen>