Add spl-downgrade field to ota metadata

In aosp/1581143 , we banned generation of SPL downgrade OTAs. However,
caller of OTA script can still force an SPL downgrade OTA by passing
--spl_downgrade flag. If this flag is specified, we propagate it to OTA
metadata so that GOTA server can properly honor this flag.

Test: th
Change-Id: Ic8cdc850d2210f4149ad9121fa4ed2e5a4f59bcc
diff --git a/tools/releasetools/ota_metadata.proto b/tools/releasetools/ota_metadata.proto
index 5da8b84..7aaca6f 100644
--- a/tools/releasetools/ota_metadata.proto
+++ b/tools/releasetools/ota_metadata.proto
@@ -105,4 +105,7 @@
   bool retrofit_dynamic_partitions = 7;
   // The required size of the cache partition, only valid for non-A/B update.
   int64 required_cache = 8;
+
+  // True iff security patch level downgrade is permitted on this OTA.
+  bool spl_downgrade = 9;
 }