Force set updatable apex to false

This is needed for flattened apexes to work. Note that the system/core
hack for overriding vendor props with system ones is required.

BoardConfigCommon.mk

@@ -3,6 +3,9 @@ DEVICE_PATH := device/peter/gsi
 # Bluetooth
 BOARD_BLUETOOTH_BDROID_BUILDCFG_INCLUDE_DIR := $(DEVICE_PATH)/bluetooth
 
+# Props
+TARGET_SYSTEM_PROP := $(DEVICE_PATH)/system.prop
+
 # Sepolicy
 SYSTEM_EXT_PRIVATE_SEPOLICY_DIRS += $(DEVICE_PATH)/sepolicy/private
 SELINUX_IGNORE_NEVERALLOWS := true

system.prop

@@ -0,0 +1 @@
+ro.apex.updatable=false