update_engine: Implement Excluder Class + Tests

Excluder persists the exclusion state for excluding certain names. This
will be used to exclude update Payloads which are contiuously faulty.

BUG=chromium:928805
TEST=FEATURES=test emerge-$B update_engine
TEST=USE="${USE} -dlc" FEATURES=test emerge-$B update_engine

Change-Id: I780a9cf2ad979833382a832e01833211ec2ccf7d
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2172074
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
Auto-Submit: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/common/constants.h b/common/constants.h
index 67519bd..248fd05 100644
--- a/common/constants.h
+++ b/common/constants.h
@@ -19,6 +19,9 @@
 
 namespace chromeos_update_engine {
 
+// The root path of all exclusion prefs.
+extern const char kExclusionPrefsSubDir[];
+
 // The root path of all DLC metadata.
 extern const char kDlcPrefsSubDir[];