Don't FinishUpdate() in BootControl*::SetActiveBootSlot()

Instead, callers call GetDynamicPartitionControl()->FinishUpdate()
directly.

This is one more step forward for keeping BootControlAndroid a thin
wrapper over the boot control HAL.

Test: builds
Test: update_engine_unittests
Change-Id: I95773ed7f230d1a666d5391bec7783e42b90bf64
diff --git a/payload_consumer/postinstall_runner_action.cc b/payload_consumer/postinstall_runner_action.cc
index a782b8f..1367649 100644
--- a/payload_consumer/postinstall_runner_action.cc
+++ b/payload_consumer/postinstall_runner_action.cc
@@ -339,7 +339,8 @@
   // steps succeeded.
   if (error_code == ErrorCode::kSuccess) {
     if (install_plan_.switch_slot_on_reboot) {
-      if (!boot_control_->SetActiveBootSlot(install_plan_.target_slot)) {
+      if (!boot_control_->GetDynamicPartitionControl()->FinishUpdate() ||
+          !boot_control_->SetActiveBootSlot(install_plan_.target_slot)) {
         error_code = ErrorCode::kPostinstallRunnerError;
       }
     } else {