Revert "Replace system_ext container with system"

This reverts commit d80efafd.

A system partition cannot have the aconfigs of other partitions.
Revive the system_ext container for system_ext aconfigs.

Bug: 385213580
Test: "adb shell aflags list" must not be changed.
Flag: EXEMPT refactor
Change-Id: I92387e6d5764c750f83bd8d266f6da969e114a72