Fix a bug when paths is a string in find_files.

When test_paths is give as a path string the utils.find_files in
test_runner.py line 230 will return wrong list of file_list due to
this bug.

Test: run ACTs in the lab
Bug: 33492041
Change-Id: Ie2bbb19d1fd9ae242f8247923835e17f73e651fc
diff --git a/acts/framework/acts/utils.py b/acts/framework/acts/utils.py
index 62f66aa..23f231d 100755
--- a/acts/framework/acts/utils.py
+++ b/acts/framework/acts/utils.py
@@ -188,6 +188,7 @@
         A list of files that match the predicate.
     """
     file_list = []
+    if isinstance(paths, str): paths = [paths]
     for path in paths:
         p = abs_path(path)
         for dirPath, subdirList, fileList in os.walk(p):