Don't initialize CBCentralManager until needed

[rohansingh]

By deferring the initialization of CBCentralManager, we are able to defer prompting for Bluetooth permission on iOS until a flutter_blue method is called. Otherwise, the Bluetooth permission prompt appears right when the app starts up.

[CLAassistant]

All committers have signed the CLA.

[qimingweng]

This is really important for any production app that wants a reasonable user experience!

[papaoloba]

You saved my life!

[qimingweng]

@pauldemarco do you have any opinion on this pull request?

[rockerhieu]

For those who want to use this PR before it is merged:

dependencies:
  flutter_blue:
    # TODO: revert to official version of flutter_blue when this is merged: https://github.com/pauldemarco/flutter_blue/pull/599
    git:
      url: https://github.com/tidbyt/flutter_blue.git
      ref: 4ed5e9980139a9521fcd4dcdd7a97084875bb7b6

And of course don't forget to subscribe to this PR.

[jasaw]

@rohansingh This PR has the unfortunate side effect where calling isOn always returns false if centralManager is initialized in the same call. This is caused by race condition of reading the Bluetooth state while the Bluetooth subsystem is still initializing.

As a workaround, I just added a small delay between the centralManager init call and the isOn call.

[ihorkozar]

@jasaw Did you insert the delay in FlitterBluePlugin.m? Can you share the implementation?