releasetools: Add unittests for GetCareMap() and AddCareMapTxtForAbOta().

With test_utils.construct_sparse_image(), creating valid sparse image is
no longer a blocker for testing these functions.

Test: python -m unittest test_add_img_to_target_files
Change-Id: Iff1f15837cc744bad52e2120f9a9ad94d7db50d5
diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index a37de66..bbb5abd 100755
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -96,7 +96,16 @@
 
 
 def GetCareMap(which, imgname):
-  """Generates the care_map for the given partition."""
+  """Returns the care_map string for the given partition.
+
+  Args:
+    which: The partition name, must be listed in PARTITIONS_WITH_CARE_MAP.
+    imgname: The filename of the image.
+
+  Returns:
+    (which, care_map_ranges): care_map_ranges is the raw string of the care_map
+    RangeSet.
+  """
   assert which in PARTITIONS_WITH_CARE_MAP
 
   simg = sparse_img.SparseImage(imgname)