Refactor String8 to std::string calls

DisplayViewport::uniqueId is now std::string, so change the calls
appropriately. Do some additional cleanups and conversions.
This almost completely removes the dependency on String8.

Test: build only
Bug: 111108021
Change-Id: Ibbb6ca59e9061954d4a5fb930ef03d42cb0230db
8 files changed