Mark some aidl_interface modules as unstable

With b/152655547, all aidl_interface modules are considered as stable
unless it is explicitly with "unstable: true". This change marks the
aidl_interface that are not used across updatable module bounraries
as unstable, so that the build system does not run the API
dumping/checking on them.

Bug: 152655547
Test: m
Change-Id: I1257c66de6dd42b2d32d47ed74cb2878f79d14fb
diff --git a/apex/aidl/Android.bp b/apex/aidl/Android.bp
index 404c632..04339e6 100644
--- a/apex/aidl/Android.bp
+++ b/apex/aidl/Android.bp
@@ -20,6 +20,7 @@
 
 aidl_interface {
     name: "statsd-aidl",
+    unstable: true,
     srcs: [
         "android/os/IPendingIntentRef.aidl",
         "android/os/IPullAtomCallback.aidl",