update_engine resize dynamic partitions during OTA.

update_engine uses device mapper to resize dynamic partitions
before opening the devices to apply the update.

* DeltaPerformer calls BootControlInterface::InitPartitionMetadata
  when parsing the update manifest. The implementation for
  BootControlAndroid::InitPartitionMetadata does the following
  if sizes for dynamic partitions are incorrect (assuming updating
  from slot A to B):
  * Load metadata from metadata slot A
  * Delete all extents of partitions at slot B (with _b suffix)
  * Add extents for partitions at slot B
  * Write metadata to metadata slot B
  * Re-map all partitions at slot B using metadata slot B with
    force_writable = true
* BootControlAndroid::GetPartitionDevice() checks device-mapper
  before returning static partitions.
* PostinstallRunnerAction::Cleanup calls BootControlInterface::Cleanup
  which unmaps all partitions at slot B.

A partition "foo" is considered dynamic if foo_a exists as a dynamic
partition OR foo_b does NOT exist as a static partition.

Bug: 110717529

Test: manual ota
Test: update_engine_unittests --gtest_filter=*BootControlAndroid*
Change-Id: I50f410b486a874242663624801c3694151bdda18
diff --git a/payload_consumer/filesystem_verifier_action.cc b/payload_consumer/filesystem_verifier_action.cc
index a5e1e61..d74d81d 100644
--- a/payload_consumer/filesystem_verifier_action.cc
+++ b/payload_consumer/filesystem_verifier_action.cc
@@ -98,13 +98,25 @@
       partition_size_ = partition.target_size;
       break;
   }
-  LOG(INFO) << "Hashing partition " << partition_index_ << " ("
-            << partition.name << ") on device " << part_path;
+
   if (part_path.empty()) {
+    if (partition_size_ == 0) {
+      LOG(INFO) << "Skip hashing partition " << partition_index_ << " ("
+                << partition.name << ") because size is 0.";
+      partition_index_++;
+      StartPartitionHashing();
+      return;
+    }
+    LOG(ERROR) << "Cannot hash partition " << partition_index_ << " ("
+               << partition.name
+               << ") because its device path cannot be determined.";
     Cleanup(ErrorCode::kFilesystemVerifierError);
     return;
   }
 
+  LOG(INFO) << "Hashing partition " << partition_index_ << " ("
+            << partition.name << ") on device " << part_path;
+
   brillo::ErrorPtr error;
   src_stream_ = brillo::FileStream::Open(
       base::FilePath(part_path),