✨ Expose CameraPickerConfig.systemOverlayStyle

[AlexV525]

Summary

• Expose CameraPickerConfig.systemOverlayStyle so callers can fully replace the SystemUiOverlayStyle applied while the picker (and its viewer) is on screen.
• Expose CameraPickerConfig.defaultSystemOverlayStyle as the picker's default: transparent status/navigation bars with light icons, systemNavigationBarDividerColor: Colors.transparent, and systemNavigationBarContrastEnforced: false.
• Apply the same AnnotatedRegion<SystemUiOverlayStyle> inside CameraPickerViewerState so the pushed viewer route stays consistent with the picker.

The previous default only set systemNavigationBarIconBrightness/statusBarIconBrightness/statusBarBrightness, leaving systemNavigationBarColor and systemNavigationBarContrastEnforced unset. On Android 9- with a virtual navigation bar, that caused the system to paint a light contrast scrim behind the picker's light icons (issue #315). Filling in the missing fields fixes that, and exposing the config lets callers override the whole style when a different platform needs a different behavior (e.g. Colors.black on devices where transparency is not honored).

AnnotatedRegion is inherently scoped, so the previous route's overlay style is automatically restored once the picker/viewer is popped.

Resolves #315

Test plan

• Reproduce [BUG] Android底部虚拟导航栏背景异常 #315 on an Android 9 device with a virtual navigation bar: previous route sets SystemUiOverlayStyle.light.copyWith(systemNavigationBarColor: Colors.transparent, systemNavigationBarIconBrightness: Brightness.dark, ...), then push the picker and confirm the navigation bar no longer renders a light scrim with invisible icons.
• Verify the picker's status/navigation bar icons remain light on Android 10+ and iOS.
• Pop the picker and confirm the previous route's overlay style is restored.
• Pass a custom CameraPickerConfig(systemOverlayStyle: ...) and confirm it is honored on both the picker and the viewer.

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