Extract TmpDirTree into a separate file

It's going to be used by other unit tests in the future.

Bug: 200136875
Change-Id: I41fa33f628a3199d9391584ecd55ee8e77aa87e7
diff --git a/Android.bp b/Android.bp
index 46db7fe..2632320 100644
--- a/Android.bp
+++ b/Android.bp
@@ -6871,6 +6871,7 @@
     name: "perfetto_src_base_test_support",
     srcs: [
         "src/base/test/test_task_runner.cc",
+        "src/base/test/tmp_dir_tree.cc",
         "src/base/test/utils.cc",
         "src/base/test/vm_test_utils.cc",
     ],