view::Surface: Add toString()

This is used for the generation of AIDL files, I'm not sure why it
hasn't broken before.

For now, it just returns the set name of the view Surface.

Bug: 340933754
Flag: EXEMPT only builds in later test, is otherwise unused
functionality, required to build
Test: new test in libgui_test

Change-Id: I86ea29a8ea7dc8270fc471a0cfd04eb38e015045