Disable use of RevocableFileDescriptor for blob reads.

We are using RevocableFileDescriptor as a precaution
to make sure when blobs get deleted, apps are not
holding onto those fds and preventing disk space from
getting freed but this might not be much
of a concern since non-system apps cannot be
persistent (so they eventually gets killed) and apps
can already fill up disk space if they want and given
that we are seeing issues with fuse reads, disabling
the use of RevocableFileDescriptor for blob reads.

Bug: 162662834
Test: atest --test-mapping apex/blobstore
Change-Id: I13a504fac154e2a3cacfb37e83a9cfec4ae80b63