trace/systrace: make start() return when tracing has started

In SystraceCollector, start() returns after executing
subprocess.Popen() for systrace. That doesn't mean that systrace is
running though, the function can return even before systrace has had a
chance to execute anything. Therefore, when you run the command
you want to trace, systrace will miss the first seconds of the
execution.

Run systrace with -u to unbuffer its stdin and wait for it to print
"Starting tracing (stop with enter)" before returning.

Fixes #403

devlib/trace/systrace.py

@@ -107,7 +107,7 @@ class SystraceCollector(TraceCollector):
         self._tmpfile = NamedTemporaryFile()
 
         # pylint: disable=attribute-defined-outside-init
-        self.systrace_cmd = '{} -o {} -e {}'.format(
+        self.systrace_cmd = 'python2 -u {} -o {} -e {}'.format(
             self.systrace_binary,
             self._tmpfile.name,
             self.target.adb_name
@@ -137,9 +137,11 @@ class SystraceCollector(TraceCollector):
         self._systrace_process = subprocess.Popen(
             self.systrace_cmd,
             stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
             shell=True,
             universal_newlines=True
         )
+        self._systrace_process.stdout.read(1)
 
     def stop(self):
         if not self._systrace_process: