Use certificate build target

With the support for multiple sources in prebuilt_etc, it is possible to
simply reference the build target instead of all the certificates it
contains.

Bug: 328313691
Test: build
Change-Id: Ic306fa22a7586d66642e30b7dfafadb91f14d65d
diff --git a/apex/Android.bp b/apex/Android.bp
index e64cef5..276d816 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -26,9 +26,7 @@
     compat_configs: ["wifi-compat-config"],
     // from build rule `cacerts_wfa`
     prebuilts: [
-        "target-cacert-wifi-674b5f5b.0",
-        "target-cacert-wifi-21125ccd.0",
-        "target-cacert-wifi-ea93cb5b.0",
+        "cacerts_wfa",
     ],
     key: "com.android.wifi.key",
     certificate: ":com.android.wifi.certificate",