Make generate_seccomp_policy.py support strace -f

This change improves the parsing of the trace files so that it supports
`strace -f`, which adds a leading [pid 12345] tag to each line.

Bug: None
Test: strace -f -e raw=all bash -c 'ls || true' 2>&1 | \
      tools/generate_seccomp_policy.py /dev/stdin
Change-Id: I5a54ddff00948b55cbcb4f0be435585b3b4cb102
2 files changed