Make file_contexts as "android:path" property

Till now, file_contexts has been treated differently that other input
src files. Now it is tagged as `android:"path"` because it is.

Bug: 144732805
Test: m
Change-Id: Ia3db7ac4e7c30d0afdcb99f33950b3075f35eed1
diff --git a/build/apex/Android.bp b/build/apex/Android.bp
index 809b8fc..70e4988 100644
--- a/build/apex/Android.bp
+++ b/build/apex/Android.bp
@@ -308,7 +308,7 @@
 art_apex_test {
     name: "com.android.art.testing",
     defaults: ["com.android.art-dev-defaults"],
-    file_contexts: "com.android.art.debug",
+    file_contexts: ":com.android.art.debug-file_contexts",
     certificate: ":com.android.art.certificate",
     tests: art_gtests,
     binaries: ["signal_dumper"], // Need signal_dumper for run-tests.