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/install_plan.cc b/payload_consumer/install_plan.cc
index 45112d6..561e294 100644
--- a/payload_consumer/install_plan.cc
+++ b/payload_consumer/install_plan.cc
@@ -103,7 +103,8 @@
       partition.source_path.clear();
     }
 
-    if (target_slot != BootControlInterface::kInvalidSlot) {
+    if (target_slot != BootControlInterface::kInvalidSlot &&
+        partition.target_size > 0) {
       result = boot_control->GetPartitionDevice(
           partition.name, target_slot, &partition.target_path) && result;
     } else {