Ensure TARGET_FLATTEN_APEX == true when ro.apex.updatable is not set

TARGET_FLATTEN_APEX and ro.apex.updatable cannot be independently set.
For a device where updating APEXes is not supported, ro.apex.updatable
should not be set (or set to false) and TARGET_FLATTEN_APEX should be
set to true. For APEX-supporting devices, it is the opposite;
ro.apex.updatable == true and TARGET_FLATTEN_APEX is false (or not set).

To ensure this relationship, TARGET_FLATTEN_APEX is by default set to
true, and overridden to false when updatable_apex.mk is inherited.

Bug: 130623080
Test: choosecombo to Pixels 2 and later; get_build_var
TARGET_FLATTEN_APEX returns false.
choosecombo to the original Pixel and other non-Pixel targets;
get_build_var TARGET_FLATTEN_APEX returns true.
choosecombo to the cuttlefish; get_build_var TARGET_FLATTEN_APEX
returns false

Merged-In: Id73a594dd9838457e68e2793122592c11a84fc83
Change-Id: Id73a594dd9838457e68e2793122592c11a84fc83
(cherry picked from commit 18411a42f0)

core/board_config.mk

@@ -534,6 +534,14 @@ else
   TARGET_VENDOR_TEST_SUFFIX :=
 endif
 
+###########################################
+# APEXes are by default flattened, i.e. non-updatable.
+# It can be unflattened (and updatable) by inheriting from
+# updatable_apex.mk
+ifeq (,$(TARGET_FLATTEN_APEX))
+TARGET_FLATTEN_APEX := true
+endif
+
 ifeq (,$(TARGET_BUILD_APPS))
 ifdef PRODUCT_EXTRA_VNDK_VERSIONS
   $(foreach v,$(PRODUCT_EXTRA_VNDK_VERSIONS),$(call check_vndk_version,$(v)))

target/product/updatable_apex.mk

@@ -18,3 +18,4 @@
 
 PRODUCT_PROPERTY_OVERRIDES := ro.apex.updatable=true
 PRODUCT_PACKAGES := com.android.apex.cts.shim.v1_prebuilt
+TARGET_FLATTEN_APEX := false