Adding "Install Other Apps" in App Info page

Added a preference link to manage external sources in the app info
screen just like other special access permissions.

Test: Manually checked that the link appears on apps once their app op
is set to allow or deny.
Also, added some basic robolectric unit tests:
make -j32 RunSettingsRoboTests

Bug: 35481942
Change-Id: If687778b7a64fb7d278508b78d06272253a98e3e

src/com/android/settings/applications/ManageApplications.java

@@ -1373,8 +1373,8 @@ public class ManageApplications extends InstrumentedPreferenceFragment
                     break;
 
                 case LIST_TYPE_MANAGE_SOURCES:
-                    holder.summary
-                            .setText(((InstallAppsState) holder.entry.extraInfo).getSummary());
+                    holder.summary.setText(ExternalSourcesDetails.getPreferenceSummary(mContext,
+                            holder.entry));
                     break;
 
                 default: