Generate aconfig flags for autogenerated system_ext partition

The aconfig flags for system_ext were previously bundled with system/,
but was recently split in https://r.android.com/3488672

Test: Verified that
out/soong/.intermediates/build/soong/fsgen/aosp_cf_x86_64_phone_generated_system_ext_image/android_common/system_ext/etc/aconfig_flags.pb
is created and is identical to make built
system_ext/etc/aconfig_flags.pb

Change-Id: I67ea7db8d4975b026986c5b0e77491793e982a2e
diff --git a/fsgen/filesystem_creator.go b/fsgen/filesystem_creator.go
index f00e491..29d5e79 100644
--- a/fsgen/filesystem_creator.go
+++ b/fsgen/filesystem_creator.go
@@ -505,6 +505,7 @@
 		}
 		fsProps.Security_patch = proptools.StringPtr(ctx.Config().PlatformSecurityPatch())
 		fsProps.Stem = proptools.StringPtr("system_ext.img")
+		fsProps.Gen_aconfig_flags_pb = proptools.BoolPtr(true)
 	case "product":
 		fsProps.Gen_aconfig_flags_pb = proptools.BoolPtr(true)
 		fsProps.Android_filesystem_deps.System = proptools.StringPtr(partitions.nameForType("system"))