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/common/fake_boot_control.h b/common/fake_boot_control.h
index 3eccc80..e71c83a 100644
--- a/common/fake_boot_control.h
+++ b/common/fake_boot_control.h
@@ -74,6 +74,13 @@
     return true;
   }
 
+  bool InitPartitionMetadata(Slot slot,
+                             const PartitionSizes& partition_sizes) override {
+    return true;
+  }
+
+  void Cleanup() override {}
+
   // Setters
   void SetNumSlots(unsigned int num_slots) {
     num_slots_ = num_slots;