Add a sample for file IO

[yschimke]

File I/O for macrobenchmark is tricky because of scoped storage. I'd suggest a good sample showing I/O patterns.

For providing input data, I ended up using FileProvider but it was non obvious.

Or maybe should be docs?

But it feels like there are problems that are common for real world macrobenchmark usage.

[MJegorovas]

Hey @yschimke, trying write a I/O benchmark and was wondering if you have any sample using this library or any tips in general about testing file I/O on android?

[yschimke]

@MJegorovas nope, sorry.

I wasn't really testing file I/O. More I was trying to test some feature that required data files to be installed for the test.