Mount /dev/fuse on /mnt/user/<userid>/<volumeid>

Since system_server cannot mount devices by itself,
add a binder interface to vold that system_server
can call to initiate this mount when required.

BUG: 135341433
Test: manual
Test: atest --test-mapping packages/providers/MediaProvider
Test: ExternalStorageHostTest DownloadProviderTests

Change-Id: If4fd02a1f1a8d921a3f96783d8c73e085c5b7ca1
diff --git a/binder/android/os/IVold.aidl b/binder/android/os/IVold.aidl
index 03fe258..b1af587 100644
--- a/binder/android/os/IVold.aidl
+++ b/binder/android/os/IVold.aidl
@@ -40,7 +40,7 @@
     void partition(@utf8InCpp String diskId, int partitionType, int ratio);
     void forgetPartition(@utf8InCpp String partGuid, @utf8InCpp String fsUuid);
 
-    void mount(@utf8InCpp String volId, int mountFlags, int mountUserId);
+    FileDescriptor mount(@utf8InCpp String volId, int mountFlags, int mountUserId);
     void unmount(@utf8InCpp String volId);
     void format(@utf8InCpp String volId, @utf8InCpp String fsType);
     void benchmark(@utf8InCpp String volId, IVoldTaskListener listener);