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