Fix bug where credstore is immediately exiting.

This CL adds the fix for the credstore failure reported in b/183253539.

Bug: 183253539
Test: CtsVerifier's Identity Credential Authentication test
Test: atest android.security.identity.cts
Change-Id: I396f36a58d260e3e081ef8a94aaf64431d0ad484
diff --git a/identity/main.cpp b/identity/main.cpp
index 08f2219..9add73c 100644
--- a/identity/main.cpp
+++ b/identity/main.cpp
@@ -53,5 +53,9 @@
     CHECK(ret == ::android::OK) << "Couldn't register binder service";
     LOG(ERROR) << "Registered binder service";
 
+    // Credstore is a single-threaded process. So devote the main thread
+    // to handling binder messages.
+    IPCThreadState::self()->joinThreadPool();
+
     return 0;
 }