fix: APEX prebuilts are disabled in platform build

This change fixes a bug that APEX prebuilts are auto-disabled in
platform builds (i.e. non-unbundled builds).

Bug: 137282010
Test: m com.android.conscrypt
check that the apex is from prebuilt directory

Change-Id: I935ef3896e80864bdcc1ca5f6fc12b63c9588c0d
(cherry picked from commit 895e224d8fad6877918ef05545d7423a30d72c2b)
diff --git a/apex/apex.go b/apex/apex.go
index e5e5860..87c54f3 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -1333,7 +1333,7 @@
 
 	// Force disable the prebuilts when we are doing unbundled build. We do unbundled build
 	// to build the prebuilts themselves.
-	forceDisable = forceDisable || !ctx.Config().UnbundledBuild()
+	forceDisable = forceDisable || ctx.Config().UnbundledBuild()
 
 	// b/137216042 don't use prebuilts when address sanitizer is on
 	forceDisable = forceDisable || android.InList("address", ctx.Config().SanitizeDevice()) ||