Set system_ext_specific on CompOSPayloadApp

This apk is packaged into com.android.compos, which is a system_ext
apex. Setting system_ext_specific on the apk ensures that its partition
in apkcerts.txt is `system_ext` when apkcerts.txt is built with both
Soong and Make.

Bug: 399788149
Test: m nothing
Change-Id: Iaea5502bcc524500002154cea6fcd92fa599771b
diff --git a/build/compos/CompOSPayloadApp/Android.bp b/build/compos/CompOSPayloadApp/Android.bp
index c6192b9..04465b3 100644
--- a/build/compos/CompOSPayloadApp/Android.bp
+++ b/build/compos/CompOSPayloadApp/Android.bp
@@ -5,5 +5,6 @@
 android_app {
     name: "CompOSPayloadApp",
     sdk_version: "current",
+    system_ext_specific: true,
     apex_available: ["com.android.compos"],
 }