Additional setup and reduced file collision

There's some additional setup needed to make sure the devices are all in
the same state, so I added instructions to the README.

I also changed the system_stats functions to use variables to store
string outputs instead of files to avoid collision when running multiple
instances of the script from the same directory.

Finally, I added cleanup at the end to remove the files created in the
process, and some general code cleanup throughout the script.

Bug: 62536649
Test: manual test on several devices
Change-Id: I843a1cefcf6bd11f28e513d735b3bf313442efd2
2 files changed