update_engine: Add PayloadState Exclusion Logic

|PayloadState| will exclude payloads based on specific update failures.
This is to prevent critical platform updates from being blocked by less
critical updates (e.g. DLCs). A layer of robustness is added in
protecting CrOS devices from falling off the update train.

Some important changes to mention:
 - Only during updates will update_engine exclude non-critical payloads
 - |OmahaRequestAction|, the current precursor |Action| to
 |OmahaResponseHandlerAction|, during a update will exclude
 faulty/excluded payloads prior to setting the |OmahaResponse| as an
 output object for suqsequent bonded |Action| to consume
 - When all payloads are excluded for an update, the |ErrorCode| will
 be indicated as |OmahaResponseInvalid| as this case is not a valid
 Omaha response update_engine should ever run into because non-critical
 updates must tag alongside a critical update

BUG=chromium:928805
TEST=FEATURES=test emerge-$B update_engine update_engine-client

Change-Id: I0551a228d0b84defb4d59966e8ed46a5d9278d60
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2190237
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Auto-Submit: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/omaha_response_handler_action.cc b/omaha_response_handler_action.cc
index 915e839..040f8e7 100644
--- a/omaha_response_handler_action.cc
+++ b/omaha_response_handler_action.cc
@@ -70,6 +70,9 @@
   }
 
   // This is the url to the first package, not all packages.
+  // (For updates): All |Action|s prior to this must pass in non-excluded URLs
+  // within the |OmahaResponse|, reference exlusion logic in
+  // |OmahaRequestAction| and keep the enforcement of exclusions for updates.
   install_plan_.download_url = current_url;
   install_plan_.version = response.version;
   install_plan_.system_version = response.system_version;