Add a neverallow rule (compile time assertion + CTS test) that
isolated_apps and untrusted_apps can't do anything else but append
to /data/anr/traces.txt. In particular, assert that they can't
read from the file, or overwrite other data which may already be
in the file.

Bug: 18340553
Bug: 27853304

(cherry picked from commit 369cf8cd)

Change-Id: Ib33e7ea0342ad28e5a89dfffdd9bc16fe54d8b3d