Revert "Replace system_ext container with system"

This reverts commit 2082a209.

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: Ib3a533493060948035c393e83a1a260fdc380624