3595 [shared_preferences] Bump platform versions to NNBD stable (cla: yes, nnbd, p: shared_preferences, platform-linux, platform-macos, platform-web, platform-windows)

3602 [shared_preferences] Bump app-facing version for NNBD stable (cla: yes, nnbd, p: shared_preferences)

3633 Make executor an instance property (cla: yes, p: shared_preferences, platform-android, waiting for tree to go green)

3639 [shared_preferences] Don’t create additional Handler when method channel is called. (cla: yes, p: shared_preferences, platform-android, waiting for tree to go green)

3684 [shared_preferences] Fix concurrent modification of the shared preferences on Android (cla: yes, p: shared_preferences, platform-android)