winscope_geometry: Make member operator== const (#1957)

Otherwise we get a warning in C++20.

```
external/perfetto/include/perfetto/ext/base/flat_hash_map.h:214:42: error: ISO C++20 considers use of overloaded operator '==' (with operand types 'perfetto::trace_processor::winscope::geometry::Rect' and 'perfetto::trace_processor::winscope::geometry::Rect') to be ambiguous despite there being a unique best viable function [-Werror,-Wambiguous-reversed-operator]
  214 |         if (tag_idx == tag && keys_[idx] == key) {
      |                               ~~~~~~~~~~ ^  ~~~
external/perfetto/src/trace_processor/importers/proto/winscope/winscope_rect_tracker.cc:38:17: note: in instantiation of member function 'perfetto::base::FlatHashMap<perfetto::trace_processor::winscope::geometry::Rect, perfetto::trace_processor::tables::WinscopeRectTable::Id, perfetto::trace_processor::winscope::RectHasher>::Insert' requested here
   38 |   return *rows_.Insert(rect, id).first;
      |                 ^
external/perfetto/src/trace_processor/importers/proto/winscope/winscope_geometry.h:47:8: note: ambiguity is between a regular call to this operator and a call with the argument order reversed
   47 |   bool operator==(const Rect& other);
      |        ^
external/perfetto/src/trace_processor/importers/proto/winscope/winscope_geometry.h:47:8: note: mark 'operator==' as const or add a matching 'operator!=' to resolve the ambiguity
1 error generated.
```

Android in-tree builds are done with C++20 now.

Also, mark other related methods as const.