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/RpcState.h b/libs/binder/RpcState.h
index 8a0610e..aacb530 100644
--- a/libs/binder/RpcState.h
+++ b/libs/binder/RpcState.h
@@ -86,7 +86,6 @@
     size_t countBinders();
     void dump();
 
-private:
     /**
      * Called when reading or writing data to a session fails to clean up
      * data associated with the session in order to cleanup binders.
@@ -105,6 +104,7 @@
      */
     void terminate();
 
+private:
     // Alternative to std::vector<uint8_t> that doesn't abort on allocation failure and caps
     // large allocations to avoid being requested from allocating too much data.
     struct CommandData {