Revert "Replace system_ext container with system"

This reverts commit d80efafdc0.

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: I92387e6d5764c750f83bd8d266f6da969e114a72

aconfig/Android.bp

@@ -20,7 +20,7 @@ package {
 aconfig_declarations {
     name: "com_android_launcher3_flags",
     package: "com.android.launcher3",
-    container: "system",
+    container: "system_ext",
     srcs: ["**/*.aconfig"],
 }
 

aconfig/launcher.aconfig

@@ -1,5 +1,5 @@
 package: "com.android.launcher3"
-container: "system"
+container: "system_ext"
 
 flag {
     name: "enable_expanding_pause_work_button"

aconfig/launcher_overview.aconfig

@@ -1,5 +1,5 @@
 package: "com.android.launcher3"
-container: "system"
+container: "system_ext"
 
 flag {
     name: "enable_grid_only_overview"

aconfig/launcher_search.aconfig

@@ -1,5 +1,5 @@
 package: "com.android.launcher3"
-container: "system"
+container: "system_ext"
 
 flag {
     name: "enable_private_space"