Allow writing to tombstone files in crash_dump_fallback.

Allows the following denials:
     avc: denied { use } for pid=9099 comm="mediacodec" path="/data/tombstones/tombstone_08" dev="sda35" ino=877473 scontext=u:r:mediacodec:s0 tcontext=u:r:tombstoned:s0 tclass=fd permissive=1
     avc: denied { append } for pid=9099 comm="mediacodec" path="/data/tombstones/tombstone_08" dev="sda35" ino=877473 scontext=u:r:mediacodec:s0 tcontext=u:object_r:tombstone_data_file:s0 tclass=file permissive=1

Bug: http://b/36156624
Test: killall -ABRT media.codec
Test: killall -ABRT media.extractor
Change-Id: I3dde1879b44e3e63c747a3ff8dd4bf213cb8afb6