Don't open appfuse files in apps.

Previously we published appfuse mount points to apps and apps open
appfuse file by themselves. We changed the design and we don't allow
apps to access appfuse mount point. Instead system server opens a file
on appfuse mount points and passes FD to apps.

The change updates apps and system server policies to adopt new design.

Bug: 29970149
Test: None
Change-Id: I0b35fee9816f61565705eecb88a472754ccffdca