make shutdown safe for double calls.

- In new shutdown sequence, volume shutdown is requested in two places:
  system_server and init.
- Make VolumeManager.shutdown safe for double calls.
  It crashed before this change.

bug: 36004738
Test: reboot and check last_kmsg for crash / vdc timeout
Change-Id: I296913959b2647b65d66553073f2032545beba57

VolumeManager.cpp

@@ -633,8 +633,13 @@ int VolumeManager::reset() {
     return 0;
 }
 
+// Can be called twice (sequentially) during shutdown. should be safe for that.
 int VolumeManager::shutdown() {
+    if (mInternalEmulated == nullptr) {
+        return 0; // already shutdown
+    }
     mInternalEmulated->destroy();
+    mInternalEmulated = nullptr;
     for (const auto& disk : mDisks) {
         disk->destroy();
     }