adb: add fdevent_run_on_main_thread.

Add a function to run a function on the main thread, to allow fdevents
that depend on a blocking function to be registered.

Bug: http://b/37869663
Test: adb_test on linux
Change-Id: I84a0b372360420b7647057297b8f437e8afa874e
diff --git a/adb/fdevent_test.h b/adb/fdevent_test.h
index f4215ae..5ca49ac 100644
--- a/adb/fdevent_test.h
+++ b/adb/fdevent_test.h
@@ -53,11 +53,11 @@
 
     size_t GetAdditionalLocalSocketCount() {
 #if ADB_HOST
-        // dummy socket installed in PrepareThread()
-        return 1;
-#else
-        // dummy socket and one more socket installed in fdevent_subproc_setup()
+        // dummy socket installed in PrepareThread() + fdevent_run_on_main_thread socket
         return 2;
+#else
+        // dummy socket + fdevent_run_on_main_thread + fdevent_subproc_setup() sockets
+        return 3;
 #endif
     }