atrace: setup saved_cmdlines_size and write only if available

Older kernels (bullhead etc) don't have saved_cmdlines_size, check and
write only if available. Also make work on non-root by setting
permissions in rc file (thanks John Reck for suggestion).

Bug: 62375794

Change-Id: Iefffc3203afd092ffb6cf420a144b38844aaee39
Signed-off-by: Joel Fernandes <joelaf@google.com>