Launcher3 is in system_ext container

As Launcher3 is installed in system_ext, com_android_launcher3_flags
is in the system_ext container. Aconfig flags must set the same
container with their declaration.

Bug: 385213580
Test: "adb shell aflags list" must not be changed.
Flag: EXEMPT refactor
Change-Id: I433756dda120de253909db663a6ecfd47d26c65d

aconfig/launcher_growth.aconfig

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