Ensure that readlink has access to /proc/self/fd

/proc/self/fd is not available when PR_DUMPABLE is set to 0
which is default for the user builds. It leads to permission
denials on readlink.

This change fixes the problem by setting PR_DUMPABLE flag to 1
for readlink and restoring it's previous value after the call.

Bug: http://b/24912743
Change-Id: I3fd179c5c6b56af96d6a15ee597024ccb15e1a13