libbinder: reverse connections

When connecting to an RPC client server, you can request to serve a
threadpool so that you can receive callbacks from it.

Future considerations:
- starting threads dynamically (likely very, very soon after this CL)
- combining threadpools (as needed)

Bug: 185167543
Test: binderRpcTest
Change-Id: I992959e963ebc1b3da2f89fdb6c1ec625cb51af4
diff --git a/libs/binder/tests/Android.bp b/libs/binder/tests/Android.bp
index 9cf433d..c7c899f 100644
--- a/libs/binder/tests/Android.bp
+++ b/libs/binder/tests/Android.bp
@@ -118,6 +118,7 @@
     host_supported: true,
     unstable: true,
     srcs: [
+        "IBinderRpcCallback.aidl",
         "IBinderRpcSession.aidl",
         "IBinderRpcTest.aidl",
     ],