Allow customization to the default car settings

For example, both multi-window and activity_on_2nd_display are disabled
with setting in package/services/Car/car_product/overlay/.../config.xml
<bool name="config_supportsMultiWindow">false</bool>

With this CL, OEM can enable multi-window by adding OEM overlay before
the default overlay above. Otherwise, they need to modify the config.xml
directly for customization.

Bug: 65256710
Test: add OEM overlay to PRODUCT_PACKAGE_OVERLAYS before car_base.mk

Change-Id: Id92186b385ffc0b750fbf600eaa62f07ff3dd097
Signed-off-by: Guobin Zhang <guobin.zhang@intel.com>
diff --git a/car_product/build/car_base.mk b/car_product/build/car_base.mk
index f637378..1e70b4a 100644
--- a/car_product/build/car_base.mk
+++ b/car_product/build/car_base.mk
@@ -17,7 +17,7 @@
 # Base platform for car builds
 # car packages should be added to car.mk instead of here
 
-PRODUCT_PACKAGE_OVERLAYS := packages/services/Car/car_product/overlay
+PRODUCT_PACKAGE_OVERLAYS += packages/services/Car/car_product/overlay
 
 PRODUCT_PACKAGES += \
     ContactsProvider \