releasetools: Fix a bug in blockimgdiff.HeapItem.

HeapItem defines __bool__(), which contains a logical error that should
return the opposite value.

Note that the bug only manifests while using Python 3, which calls
__bool__(). With Python 2, `if x:` or bool(x) actually calls
x.__nonzero__() or x.__len__(). If a class defines neither __len__() nor
__nonzero__(), as the case in HeapItem, it always returns True.

Test: python -m unittest test_blockimgdiff
Test: python3 -m unittest test_blockimgdiff
Test: Generate an incremental non-A/B OTA package successfully.
Change-Id: Ibe8430e0b495a7d2f430cfffb716d2536ffb53d2

tools/releasetools/blockimgdiff.py

@@ -237,15 +237,23 @@ class Transfer(object):
 class HeapItem(object):
   def __init__(self, item):
     self.item = item
-    # Negate the score since python's heap is a min-heap and we want
-    # the maximum score.
+    # Negate the score since python's heap is a min-heap and we want the
+    # maximum score.
     self.score = -item.score
+
   def clear(self):
     self.item = None
+
   def __bool__(self):
-    return self.item is None
+    return self.item is not None
+
+  # Python 2 uses __nonzero__, while Python 3 uses __bool__.
+  __nonzero__ = __bool__
+
+  # The rest operations are generated by functools.total_ordering decorator.
   def __eq__(self, other):
     return self.score == other.score
+
   def __le__(self, other):
     return self.score <= other.score