crash_dump_fallback: allow dumpstate:pipe_file write.

It appears that selinux requires the write permission to receive
a writable pipe from dumpstate, for unclear reasons. Add the permission
for now.

Bug: http://b/62297059
Test: dumpstate
Merged-In: I0f25682177115aacd5c2203ddc0008228b0380ad
Change-Id: I0f25682177115aacd5c2203ddc0008228b0380ad
(cherry picked from commit 7aa08523)