SEPolicy: Allow app / system_server to write to dumpstate pipes.

tombstoned allows dumpstate to install "intercepts" to java trace
requests for a given process. When an "intercept" is installed, all
trace output is redirected to a pipe provided by dumpstate instead
of the default location (usually in /data/anr or /data/tombstone).

Note that these processes are already granted "write" and "getattr"
on dumpstate:fifo_file in order to communicate with dumpstate; this
change adds "append" to the existing set of permissions.

Bug: 32064548
Test: manual
Change-Id: Iccbd78c59071252fef318589f3e55ece51a3c64c