[google_maps_flutter] Fix TileOverlay cloning

[stuartmorgan]

TileOverlay was not copying its TileProvider. During the NNBD migration,
map object updates were refactored to share code, and in that
refactoring TileOverlay update construction was aligned with other map
objects to use clones, so certain operations involving TileOverlays
started dropping the TileProvider.

Fixes flutter/flutter#77500

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[cyanglaz]

LGTM