libbase: fix the way to find temp dir.

Tests running in app context can't access /data/local/tmp,
so try current directory if /data/local/tmp is not accessible.

Bug: http://b/18790309
Test: run unit test in app context and shell context.
Change-Id: If66fe8f0ac3edb3a32a2a2a50a524364f818a58b

base/test_utils.cpp

@@ -57,7 +57,13 @@ char* mkdtemp(char* template_name) {
 
 static std::string GetSystemTempDir() {
 #if defined(__ANDROID__)
-  return "/data/local/tmp";
+  const char* tmpdir = "/data/local/tmp";
+  if (access(tmpdir, R_OK | W_OK | X_OK) == 0) {
+    return tmpdir;
+  }
+  // Tests running in app context can't access /data/local/tmp,
+  // so try current directory if /data/local/tmp is not accessible.
+  return ".";
 #elif defined(_WIN32)
   char tmp_dir[MAX_PATH];
   DWORD result = GetTempPathA(sizeof(tmp_dir), tmp_dir);