Update atrace to work with tracefs, not debugfs

When tracefs is mounted, tracing is under /sys/kernel/tracing not
/sys/kernel/debug/tracing. Modify atrace to detect this situation and
get the right files.

Bug: 31856701
Test: Run atrace, check output is same where ever tracefs is mounted
Change-Id: I78df0da91d9f56c56077d1b8b9dba7b0126b2d8d
2 files changed