atrace: Set saved_cmdline_size to 8192

This increases the size of the table tracking comms and pids and
prevents overwriting them easily.

Bug: 38167231

Change-Id: I69b9000e4f9d0c2c7969efd38e02d8c1bffdad5c
Signed-off-by: Joel Fernandes <joelaf@google.com>