Revert "Treat system_ext as system container"

This reverts commit 01efb834208f02892bf6b3d316c02fd44ea91af3,
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
diff --git a/android/aconfig_providers.go b/android/aconfig_providers.go
index 205b855..7185a89 100644
--- a/android/aconfig_providers.go
+++ b/android/aconfig_providers.go
@@ -238,8 +238,7 @@
 	} else if base.ProductSpecific() {
 		container = "product"
 	} else if base.SystemExtSpecific() {
-		// system_ext and system partitions should be treated as one container
-		container = "system"
+		container = "system_ext"
 	}
 
 	return container
@@ -255,8 +254,7 @@
 	} else if commonInfo.ProductSpecific {
 		container = "product"
 	} else if commonInfo.SystemExtSpecific {
-		// system_ext and system partitions should be treated as one container
-		container = "system"
+		container = "system_ext"
 	}
 
 	return container