Suppress [CallbackInterface] metalava rule for mainline modules.

This rule is already suppressed for main framework code in
framework/Android.bp, but mainline uses a different set of metalava
flags.

Android API council says this is the right thing to do for libraries
which run only on Java 8+, which is true of all mainline modules and
framework code.

Bug: 172180369
Test: AppSearch BatchResultCallback now builds
Change-Id: If81cda012b4627c8018ae54a0f0932ee39d6fd96
diff --git a/apex/Android.bp b/apex/Android.bp
index 0a535a8..4e80acb 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -15,6 +15,7 @@
 mainline_stubs_args =
     "--error UnhiddenSystemApi " +
     "--hide BroadcastBehavior " +
+    "--hide CallbackInterface " +
     "--hide DeprecationMismatch " +
     "--hide HiddenSuperclass " +
     "--hide HiddenTypedefConstant " +