Revert "Replace system_ext container with system"

This reverts commit 6e1b274f.

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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:1337a88d112b005f897b3745b5475cf7d4764db7)
Merged-In: I8f52d5f86e6ffaccd624846fbe1ba0a8fc842af6

Change-Id: I8f52d5f86e6ffaccd624846fbe1ba0a8fc842af6