Disable apex update for GSI

Currently, GSI does not include apex support and TARGET_FLATTEN_APEX is true.
This will cause issues for vendors with apex support (e.g. Cuttlefish).
This change set ro.apex.updatable to false in product to override that sets
in vendor.

Bug: 135411972
Bug: 134673003
Test: $ lunch aosp_x86-userdebug; m -j; emulator
      $ lunch aosp_cf_x86_phone-userdebug
      # Replace system.img in super.img with GSI
      # The resulted CF could boot and browse the web successfully.

Change-Id: I08fd7a1b254aac276926329e064c35b714764936