tombstoned: allow append to anr_data_file.

Fix the following denial:

tombstoned: type=1400 audit(0.0:563): avc: denied { append } for path="/data/anr/traces.txt" dev="sda35" ino=679941 scontext=u:r:tombstoned:s0 tcontext=u:object_r:anr_data_file:s0 tclass=file permissive=0

Bug: http://b/34472671
Test: mma
Change-Id: Iab5fbaf50888aa0f195841cb7e718ff393e526dd