Include system_ext to load aconfig flags

system_ext flags must be loaded from the system_ext container.
Add the system_ext container to the container list.

Bug: 400955438
Bug: 385213580
Test: adb shell aflags enable <flag>
Test: Use Flag Flipper
Change-Id: I73c0f7f706cc9f48a7129d7e6f9e8d9ae4225b35