libsnapshot: Rename and clarify some methods in ICowOpIter.

Done and RDone are not exactly symmetrical, so rename them to match
their underlying STL behavior. Done becomes "AtEnd" and RDone becomes
"AtBegin".

Bug: 280529365
Test: builds
Change-Id: I876f858a7bef84d8b37c1c822fb42941fbcded25
10 files changed