Revert "Treat system_ext as system container"

This reverts commit 01efb834,
but also has some changes for the CLs added after the commit.

Reason for revert: Revive system_ext container.
System container must not include system_ext aconfigs.

Bug: 385213580
Test: lunch gsi_x86_64_soong_system-trunk_staging-userdebug && m
      diff the aconfig_flags.pb
Change-Id: I5ffe699595b5734d3555eb248cf36dedfc2d2105