Default PRODUCT_CGROUP_V2_SYS_APP_ISOLATION_ENABLED to true

This variable can still be overridden to false, however we plan to add
a dependency on it being enabled in the near future.

Test: m
Test: cuttlefish boot with and without a device override
Bug: 377579705
Change-Id: I31f919f0c29fc606bc33dd843171a36924275363