Implement ExtentLess correctly

If two extents start on the same block but have different number of
blocks, current impl of ExtentLess will consider them equal.

Test: th
Change-Id: Ief147f9d0a91b3dcd7f8ccc690f3aea972cbeabb
diff --git a/payload_consumer/extent_map.h b/payload_consumer/extent_map.h
index aeb7066..65f1bcd 100644
--- a/payload_consumer/extent_map.h
+++ b/payload_consumer/extent_map.h
@@ -22,6 +22,7 @@
 #include <utility>
 
 #include "update_engine/common/utils.h"
+#include "update_engine/payload_generator/extent_utils.h"
 #include "update_engine/payload_generator/extent_ranges.h"
 #include "update_engine/update_metadata.pb.h"
 
@@ -35,7 +36,7 @@
 class ExtentMap {
  public:
   bool AddExtent(const Extent& extent, T&& value) {
-    if (Get(extent)) {
+    if (set_.OverlapsWithExtent(extent)) {
       return false;
     }
     const auto& [it, inserted] = map_.insert({extent, std::forward<T>(value)});