Capture systrace buffers.

When tracing is enabled (/sys/kernel/debug/tracing/tracing_on = 1),
dumpstate will run 'atrace --async_dump' and redirect its output to a
systrace.txt entry in the zip file.

BUG: 27419521
Change-Id: Ia6de46a691b25febac31331fe0aa8701c9a84ebb
4 files changed