Move ScreenshotProxyService, etc to proxy/*, unify naming

The service implementation is internal and usage should only be
through the injectable interface 'ScreenshotProxy'. This also unifies
the naming to be consistent with the interface and client wrapper.

Flag: NONE straight rename no functional changes
Test: N/A
Bug: 339223847

Change-Id: Ie06aeabfb67f93fddd01981865a03a88c67fc625