Performance optimizations for new insets

- Prevent unnecesary dispatchApplyInsets caused by legacy system
also requesting inset changes
- Make insetsModified oneway. It's safe to do so because we
absolutely don't care about interleaving with other WindowSession
methods.
- Do not trigger layout if nothing relevant has changed
- Only trigger requestFitSystemWindows if state actually changed

Test: Systrace. Automated perf test will be added
Bug: 151865131
Change-Id: I24944875e739e4a74606e3a02bbf14585c1c13db