Move ro.apex.updatable to the system partition

The decision to support updatable APEX or not used to be SoC-specific
because updatable APEX (aka non-flattened APEX) requires some kernel
feature support like loopback device. Kernel was considered as part of
BSP then. Therefore, ro.apex.updatable property was in the vendor
partition.

However, with GKI, kernel is no longer SoC-specific. And most APEXes are
installed to the system partition, which means that the decision affects
how the system partition is built. Thus, this CL moves the property to
the system partition. This enables some partners who have been using
flattened APEX to be able to upgrade to non-flattened APEX without
having to upgrade the vendor partition.

Bug: 281007951
Test: check system/build.prop
Change-Id: I81874076862f6047b9daa14518b95adcb5275064
diff --git a/target/product/updatable_apex.mk b/target/product/updatable_apex.mk
index d606e00..c19982b 100644
--- a/target/product/updatable_apex.mk
+++ b/target/product/updatable_apex.mk
@@ -20,7 +20,7 @@
   # com.android.apex.cts.shim.v1_prebuilt overrides CtsShimPrebuilt
   # and CtsShimPrivPrebuilt since they are packaged inside the APEX.
   PRODUCT_PACKAGES += com.android.apex.cts.shim.v1_prebuilt
-  PRODUCT_VENDOR_PROPERTIES := ro.apex.updatable=true
+  PRODUCT_SYSTEM_PROPERTIES := ro.apex.updatable=true
   TARGET_FLATTEN_APEX := false
   # Use compressed apexes in pre-installed partitions.
   # Note: this doesn't mean that all pre-installed apexes will be compressed.