Add unittest for CleanupPreviousUpdateAction

We had a severe bug in CleanupPreviousUpdateAction, undetected for a
while. Added unittest to cover the bug so we don't regress.

Bug: 169436297
Bug: 178637306

Test: th
Change-Id: I40fc97a5c771ec001896ecd8f6d46b551cc7724c
diff --git a/Android.bp b/Android.bp
index a4b7978..7cdd64c 100644
--- a/Android.bp
+++ b/Android.bp
@@ -763,6 +763,7 @@
 
     srcs: [
         "aosp/apex_handler_android_unittest.cc",
+        "aosp/cleanup_previous_update_action_unittest.cc",
         "aosp/dynamic_partition_control_android_unittest.cc",
         "aosp/update_attempter_android_unittest.cc",
         "certificate_checker_unittest.cc",