binderRpcTest: use TMPDIR

There was some discussion in review that my existing try and check
solution was messy. I'm not sure why I didn't use this.

Bug: 167966510
Test: run binderRpcTest on device & host
Change-Id: I594a410cf3712bcc22b9713fa3d82757e33f08d1
diff --git a/libs/binder/tests/binderRpcTest.cpp b/libs/binder/tests/binderRpcTest.cpp
index a51c987..3340293 100644
--- a/libs/binder/tests/binderRpcTest.cpp
+++ b/libs/binder/tests/binderRpcTest.cpp
@@ -199,13 +199,8 @@
 
 static std::string allocateSocketAddress() {
     static size_t id = 0;
-    static bool gUseTmp = access("/tmp/", F_OK) != -1;
-
-    if (gUseTmp) {
-        return "/tmp/binderRpcTest_" + std::to_string(id++);
-    } else {
-        return "/dev/binderRpcTest_" + std::to_string(id++);
-    }
+    std::string temp = getenv("TMPDIR") ?: "/tmp";
+    return temp + "/binderRpcTest_" + std::to_string(id++);
 };
 
 struct ProcessConnection {