feat: refactor Health class to remove singleton pattern and enable dependency injection for DeviceInfoPlugin

[iarata]

This PR removes the singleton pattern and allows for dependency injection for the DeviceInfoPlugin. Additionally, it updates the example application to reflect these changes.

• Dependency Injection for DeviceInfoPlugin:

  • The Health class is no longer a singleton and the Health() factory constructor is removed.
  • The Health class now accepts an optional DeviceInfoPlugin dependency through its constructor to facilitate easier mocking during unit tests. [1] [2]

• Global Instance Initialization:

  • A global instance of the Health class is created and used throughout the example application instead of calling Health() multiple times.
  • Updated all method calls in _HealthAppState to use the global health instance. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

• Added mocktail as a development dependency in pubspec.yaml to support mocking in unit tests.

[bardram]

Looks good.