Track current user without calling into AMS.

From a layering point-of-view, StorageManager is below ActivityManager
and it shouldn't be calling upwards, since that risks locking
inversions.  Instead, track the current user through the existing
SystemService callbacks.

Bug: 28598527
Change-Id: I02d3277eb26288d3b4e1897f4c9318a6288c9551

services/core/java/com/android/server/MountService.java

@@ -177,6 +177,11 @@ class MountService extends IMountService.Stub
             }
         }
 
+        @Override
+        public void onSwitchUser(int userHandle) {
+            mMountService.mCurrentUserId = userHandle;
+        }
+
         @Override
         public void onUnlockUser(int userHandle) {
             mMountService.onUnlockUser(userHandle);
@@ -320,6 +325,8 @@ class MountService extends IMountService.Stub
     @GuardedBy("mLock")
     private String mMoveTargetUuid;
 
+    private volatile int mCurrentUserId = UserHandle.USER_SYSTEM;
+
     private VolumeInfo findVolumeByIdOrThrow(String id) {
         synchronized (mLock) {
             final VolumeInfo vol = mVolumes.get(id);
@@ -1285,7 +1292,7 @@ class MountService extends IMountService.Stub
                 vol.mountFlags |= VolumeInfo.MOUNT_FLAG_VISIBLE;
             }
 
-            vol.mountUserId = ActivityManager.getCurrentUser();
+            vol.mountUserId = mCurrentUserId;
             mHandler.obtainMessage(H_VOLUME_MOUNT, vol).sendToTarget();
 
         } else if (vol.type == VolumeInfo.TYPE_PRIVATE) {