3548 [google_maps_flutter] Migrate to NNBD (cla: yes, nnbd, p: google_maps_flutter)

3590 [google_maps_flutter] fixed a small bug in example app. (cla: yes, p: google_maps_flutter)

3598 [google_maps_flutter] Bump platform interface version for NNBD stable (cla: yes, nnbd, p: google_maps_flutter)

3623 [google_maps_flutter] Bump app-facing version for NNBD stable (cla: yes, nnbd, p: google_maps_flutter, waiting for tree to go green)

3625 [google_maps_flutter] embedded_views_preview not required since v1.0.0 (cla: yes, p: google_maps_flutter, platform-ios)