Fix perfboot.py to work with the recent change.

device.shell() was changed to return a (stdout, stderr) tuple in
the following CL.

https://android-review.googlesource.com/170420

This CL fixes perfboot.py to work with the return value.

Change-Id: I85d72fc63c1f96257fb19c3051482991a7b4049d
1 file changed