config: Build NavigationBarMode2ButtonOverlay

This got removed from PRODUCT_PACKAGES in frameworks/base, but it seems
like it still exists.

Change-Id: I8fe80c9c5a386ae1877f7c117ddc76ff366304a3
diff --git a/config/common_mobile.mk b/config/common_mobile.mk
index ef8a5bc..a4b2eff 100644
--- a/config/common_mobile.mk
+++ b/config/common_mobile.mk
@@ -25,7 +25,8 @@
 
 # Customizations
 PRODUCT_PACKAGES += \
-    LineageNavigationBarNoHint
+    LineageNavigationBarNoHint \
+    NavigationBarMode2ButtonOverlay
 
 # Media
 PRODUCT_SYSTEM_DEFAULT_PROPERTIES += \