TaskRunner starts the background thread only when needed

Test: libhidl_test
Test: hidl_test
Test: boots
Change-Id: I5b6de5cd4a475b0653102e8004953ba008288ab1
diff --git a/test_main.cpp b/test_main.cpp
index 308a8cc..7859156 100644
--- a/test_main.cpp
+++ b/test_main.cpp
@@ -257,6 +257,7 @@
 TEST_F(LibHidlTest, TaskRunnerTest) {
     using android::hardware::details::TaskRunner;
     TaskRunner tr;
+    tr.start(1 /* limit */);
     bool flag = false;
     tr.push([&] {
         usleep(1000);