updatable_apex.mk is not inherited from  mainline_system.mk

mainline_system.mk is inherited by not only Pixels but also products
like GSI and sdk_gphone where kernel isn't capable of supporting
updatable APEX.

Stop inheriting updatable_apex.mk from mainline_system.mk and let each
product to decide whether or not to inherit updatable_apex.mk.

Also, TARGET_FLATTEN_APEX is not set to true for GSI as it is the
default setting.

Bug: 132664085
Test: choosecombo 1 aosp_arm64 userdebug; get_build_var
TARGET_FLATTEN_APEX shows true
choosecombo 1 sdk_gphone userdebug; get_build_var TARGET_FLATTEN_APEX
shows true

Change-Id: I3c4e1e4b2bbebd675904d148dd18acb57ecbdfe9
diff --git a/target/board/BoardConfigGsiCommon.mk b/target/board/BoardConfigGsiCommon.mk
index 96fd07b..a26b2b7 100644
--- a/target/board/BoardConfigGsiCommon.mk
+++ b/target/board/BoardConfigGsiCommon.mk
@@ -52,7 +52,3 @@
 
 # Disable 64 bit mediadrmserver
 TARGET_ENABLE_MEDIADRM_64 :=
-
-# Ordinary (non-flattened) APEX may require kernel changes. For maximum compatibility,
-# use flattened APEX for GSI
-TARGET_FLATTEN_APEX := true
diff --git a/target/product/mainline_system.mk b/target/product/mainline_system.mk
index 54a9625..4b4bfda 100644
--- a/target/product/mainline_system.mk
+++ b/target/product/mainline_system.mk
@@ -18,8 +18,6 @@
 $(call inherit-product, $(SRC_TARGET_DIR)/product/handheld_system.mk)
 $(call inherit-product, $(SRC_TARGET_DIR)/product/telephony_system.mk)
 $(call inherit-product, $(SRC_TARGET_DIR)/product/languages_default.mk)
-# Enable updating of APEXes
-$(call inherit-product, $(SRC_TARGET_DIR)/product/updatable_apex.mk)
 # Add adb keys to debuggable AOSP builds (if they exist)
 $(call inherit-product-if-exists, vendor/google/security/adb/vendor_key.mk)
 
@@ -110,6 +108,9 @@
 # Enable dynamic partition size
 PRODUCT_USE_DYNAMIC_PARTITION_SIZE := true
 
+PRODUCT_PACKAGES += \
+    com.android.apex.cts.shim.v1_prebuilt
+
 PRODUCT_NAME := mainline_system
 PRODUCT_BRAND := generic
 
diff --git a/target/product/updatable_apex.mk b/target/product/updatable_apex.mk
index 038f66e..a9f4baf 100644
--- a/target/product/updatable_apex.mk
+++ b/target/product/updatable_apex.mk
@@ -17,5 +17,4 @@
 # Inherit this when the target needs to support updating APEXes
 
 PRODUCT_PROPERTY_OVERRIDES := ro.apex.updatable=true
-PRODUCT_PACKAGES := com.android.apex.cts.shim.v1_prebuilt
 TARGET_FLATTEN_APEX := false