Check for valid paths when getVolumeList() returns invisible volumes.

The FLAG_INCLUDE_INVISIBLE was created to let SM.getStorageVolumes()
return mounted volumes (like USB drives), but in the current form it
might return invalid volumes. For example, when a SD Card is adopted as
an internal storage, it will return 2 volumes instead of one, the "bad"
one being:

VolumeInfo{emulated}:
     type=EMULATED diskId=null partGuid=null mountFlags=0 mountUserId=-1
     state=UNMOUNTED
     fsType=null fsUuid=null fsLabel=null
     path=null internalPath=null

As such, we need to filter out those volumes that don't have a path.

BUG: 29250241
Change-Id: I3f84b53eac98f31f918d843c70f9d42983e2a438

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

@@ -3031,7 +3031,8 @@ class MountService extends IMountService.Stub
                 if (forWrite) {
                     match = vol.isVisibleForWrite(userId);
                 } else {
-                    match = vol.isVisibleForRead(userId) || includeInvisible;
+                    match = vol.isVisibleForRead(userId)
+                            || (includeInvisible && vol.getPath() != null);
                 }
                 if (!match) continue;