Undo Utils dependency on VolumeManager

I want to use Utils in another executable, so breaking this link.

Bug: 25861755
Test: compiles (and boots, though that doesn't exercise changed code)
Change-Id: I6bb447453bb370fefb7f2f3aceb459428bdee6a7
diff --git a/VolumeManager.cpp b/VolumeManager.cpp
index c1d51d9..1847ab1 100644
--- a/VolumeManager.cpp
+++ b/VolumeManager.cpp
@@ -62,8 +62,6 @@
 using android::base::StringPrintf;
 using android::base::unique_fd;
 
-bool VolumeManager::shutting_down = false;
-
 static const char* kPathUserMount = "/mnt/user";
 static const char* kPathVirtualDisk = "/data/misc/vold/virtual_disk";
 
@@ -535,14 +533,14 @@
     if (mInternalEmulated == nullptr) {
         return 0; // already shutdown
     }
-    shutting_down = true;
+    android::vold::sSleepOnUnmount = false;
     mInternalEmulated->destroy();
     mInternalEmulated = nullptr;
     for (const auto& disk : mDisks) {
         disk->destroy();
     }
     mDisks.clear();
-    shutting_down = false;
+    android::vold::sSleepOnUnmount = true;
     return 0;
 }