Create user directory on emulated storage.

When mounting a primary external storage device that is multi-user
aware, ensure that the user-specific directory actually exists before
moving forward.

Bug: 22472026
Change-Id: I33c8eed261a9c0d5acedd5be6133ed9990679d08
diff --git a/VolumeManager.cpp b/VolumeManager.cpp
index 6710b0c..ddb7517 100755
--- a/VolumeManager.cpp
+++ b/VolumeManager.cpp
@@ -416,6 +416,7 @@
     std::string source(mPrimary->getPath());
     if (mPrimary->getType() == android::vold::VolumeBase::Type::kEmulated) {
         source = StringPrintf("%s/%d", source.c_str(), userId);
+        fs_prepare_dir(source.c_str(), 0755, AID_ROOT, AID_ROOT);
     }
 
     std::string target(StringPrintf("/mnt/user/%d/primary", userId));