Improve readability of apk size reference files

[radekdoulik]

So far we used csv files (like https://github.com/xamarin/xamarin-android/blob/master/tests/apk-sizes-reference/TestResult-Xamarin.Forms_Tests-values-Release.csv), where we have long lines and it is not very clear, which number belongs to which header. (it is a bit better when viewed on github, compared to reading the plain text)