Allow on device spl downgrade check to be bypassed

Certain OEMs bootloader implementation allow SPL downgrades, therefore
provide an option to bypass on device SPL checks.

Test: th
Bug: 306271739
Change-Id: Iee70ac5bc716c4fed9869928ba6e41c00eab5fd5
diff --git a/aosp/update_attempter_android.cc b/aosp/update_attempter_android.cc
index e52f26a..d7cb2d2 100644
--- a/aosp/update_attempter_android.cc
+++ b/aosp/update_attempter_android.cc
@@ -311,6 +311,9 @@
   install_plan_.powerwash_required =
       GetHeaderAsBool(headers[kPayloadPropertyPowerwash], false);
 
+  install_plan_.spl_downgrade =
+      GetHeaderAsBool(headers[kPayloadPropertySplDowngrade], false);
+
   if (!IsProductionBuild()) {
     install_plan_.disable_vabc =
         GetHeaderAsBool(headers[kPayloadDisableVABC], false);