simpleperf: avoid selinux denials.

1. Don't stat() entries in /proc directory if not necessary.
2. Don't read tracing events in debugfs if not root.

Bug: http://b/30981595
Change-Id: Ic695671357dad936d60f4e6d573ce4452dec217c
Test: run `simpleperf list` and check kmsg.
5 files changed