releasetools: Really allow removing partitions

- Add hashtree_info to EmptyImage so that BlockDifference.Compute()
can accept EmptyImage() as target image, which is the case when
a partition is removed.

- BlockDifference also checks source_info_dict to determine
whether a partition is dynamic. When a partition is removed,
its name does not appear in target_info_dict.

- Add tests to ensure DynamicPartitionDifference() still works.

Test: DynamicPartitionDifferenceTest
Test: test_blockimgdiff

Change-Id: Iadb1db075f5dc344db6d5ade358c83b01231e443
diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index 456f791..e5a9050 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -84,6 +84,7 @@
     self.extended = RangeSet()
     self.total_blocks = 0
     self.file_map = {}
+    self.hashtree_info = None
 
   def RangeSha1(self, ranges):
     return sha1().hexdigest()