Revert "Don't inherit updatable_apex from mainline_system"

This reverts commit 9af722d59e2e4be249c246ff85cbdde77bb9c925.

Reason for revert: kernels in the internal master are all patched
to support APEX update.

Bug: 131162245
Test: test camera on Pixel 2019
Change-Id: I9e8630993a0be7db473cf65f9e38f5bc7f7e52de
diff --git a/target/product/mainline_system.mk b/target/product/mainline_system.mk
index 87393d4..66207df 100644
--- a/target/product/mainline_system.mk
+++ b/target/product/mainline_system.mk
@@ -19,7 +19,7 @@
 $(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, $(SRC_TARGET_DIR)/product/updatable_apex.mk)
+$(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)