Force generic system image to load dex pre-opt from system

Generic system image always generates dex pre-opt in system image,
to improve the time of first boot.

But the property ro.cp_system_other_odex could be defined in vendor
image, so we disable the property in generic system image to force
dex pre-opt be loaded from system image.

Bug: 62645784
Test: Test the patch on a A/B project with GSI, confirm the dex
      pre-opt is loaded from GSI.

Change-Id: I71b4a5bed517841b5f889d21d73429fa91da25ba

target/board/treble_system.prop

@@ -0,0 +1,2 @@
+# GSI always generate dex pre-opt in system image
+ro.cp_system_other_odex=0