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.cpp b/adb/fdevent_test.cpp
index bdb973a..86e0209 100644
--- a/adb/fdevent_test.cpp
+++ b/adb/fdevent_test.cpp
@@ -173,3 +173,24 @@
     std::thread thread(InvalidFdThreadFunc);
     thread.join();
 }
+
+TEST_F(FdeventTest, run_on_main_thread) {
+    std::vector<int> vec;
+
+    PrepareThread();
+    std::thread thread(fdevent_loop);
+
+    for (int i = 0; i < 100; ++i) {
+        fdevent_run_on_main_thread([i, &vec]() {
+            check_main_thread();
+            vec.push_back(i);
+        });
+    }
+
+    TerminateThread(thread);
+
+    ASSERT_EQ(100u, vec.size());
+    for (int i = 0; i < 100; ++i) {
+        ASSERT_EQ(i, vec[i]);
+    }
+}