Refactor SkFILEStream for clarity. am: eb91df1da1 am: afc6c5e3b8 am: c6493aae9a

Original change: https://android-review.googlesource.com/c/platform/external/skia/+/1426988

Change-Id: Ib50e3a15626c60c28363d8cb9e963a501d630990