Add [un]mapPartitions APIs to dynamic partition control

During postinstall stage, update_engine needs all target
partitions(system_b, vendor_b) to be mounted. We add 2 APIs to dynamic
partition control, 1 for mounting all partitions, and UnmapAllPartitions for unmap all partitions.

Test: Treehugger
Bug: 168554689
Change-Id: I1047851f6cdae7f39f50bd1d50e6d3bfc0d9a7f5
diff --git a/dynamic_partition_control_android.h b/dynamic_partition_control_android.h
index 9bffb59..4f3964b 100644
--- a/dynamic_partition_control_android.h
+++ b/dynamic_partition_control_android.h
@@ -90,6 +90,8 @@
       const std::optional<std::string>& source_path,
       bool is_append) override;
 
+  bool UnmapAllPartitions() override;
+
  protected:
   // These functions are exposed for testing.
 
@@ -201,11 +203,14 @@
       const DeltaArchiveManifest& manifest,
       bool delete_source);
 
+  bool MapAllPartitions() override;
+
  private:
   friend class DynamicPartitionControlAndroidTest;
   friend class SnapshotPartitionTestP;
 
-  void UnmapAllPartitions();
+  std::optional<base::FilePath> GetSuperDevice();
+
   bool MapPartitionInternal(const std::string& super_device,
                             const std::string& target_partition_name,
                             uint32_t slot,