Add IsDynamicPartition API to easily check if a partition is dynamic

Test: treehugger
Change-Id: I655e4d6114a25c58857679985cd5e08581c0faab
diff --git a/common/dynamic_partition_control_stub.cc b/common/dynamic_partition_control_stub.cc
index 31975b3..2c6bb1b 100644
--- a/common/dynamic_partition_control_stub.cc
+++ b/common/dynamic_partition_control_stub.cc
@@ -111,4 +111,9 @@
   return false;
 }
 
+bool DynamicPartitionControlStub::IsDynamicPartition(
+    const std::string& part_name) {
+  return false;
+}
+
 }  // namespace chromeos_update_engine