Add a placeholder implementation for non-mobile platforms

[swift-kim]

Sets the default value of ReactiveBlePlatform.instance to an instance of _PlaceholderImplementation, rather than ReactiveBleMobilePlatform (current) which is for mobile platforms only. This is necessary so that federated implementations of this plugin (endorsed or not) can be added for non-mobile (desktop) platforms.

The same approach is being used by 1st-party plugin developers as they have decided to remove default method channel implementations from their platform interface packages. For example: https://github.com/flutter/plugins/blob/2ce625f1a87e4b738f63a6f1f7e3e040e7c0b87f/packages/video_player/video_player_platform_interface/lib/video_player_platform_interface.dart#L22

After this change, a new platform implementation can be added by providing a static registerWith method that is used to register the plugin for that platform on app startup (documentation). Here is the example:

• https://github.com/flutter/plugins/blob/2ce625f1a87e4b738f63a6f1f7e3e040e7c0b87f/packages/image_picker/image_picker_windows/pubspec.yaml#L16
• https://github.com/flutter/plugins/blob/2ce625f1a87e4b738f63a6f1f7e3e040e7c0b87f/packages/image_picker/image_picker_windows/lib/image_picker_windows.dart#L51-L54

I personally need this change to add a Tizen platform implementation of this plugin for our project. Any comments will be appreciated. Thank you!

[swift-kim]

The following PRs will need an update after this change.

• Federated implementations #599
• Implement web/mac/windows/linux support  #544
• Implement Linux support #347

[swift-kim]

Should I increase the version numbers in pubspec.yaml's and update the CHANGELOG files myself if I want to release this change, or is the release done on a regular basis?

[swift-kim]

@Taym95 Hi. If you have some time, could you please take a look at this change and give me some advice?

[Taym95]

@Taym95 Hi. If you have some time, could you please take a look at this change and give me some advice?

I will review it today, sorry for not responding quickly

[swift-kim]

@Taym95 Hi. Could you review and give some comments?

[Taym95]

Thank you for contribution, this will be helpful for endorsed package indeed.

[Taym95]

@Taym95 Hi. Could you review and give some comments?

please update your branch

[swift-kim]

@Taym95 Sure. I hope there will be a version bump soon after merge so that this change can be released.

[Taym95]

@Taym95 Sure. I hope there will be a version bump soon after merge so that this change can be released.

Yes