v3.1.0 Update native dependencies

What’s New:

• Updated native SDK versions: Android to 7.2.0, iOS to 5.8.

Full Changelog: v3.0.0...v3.1.0

v3.0.0 Add prefix to public classes

What’s New:

• Added prefix AppMetrica to all classes to remove possible conflicts with other plugins.
• Added parameter AppMetricaConfig.flutterCrashReporting. Flutter crashes will not be handled if it is set to false.
• Upgrade decimal version to 3.0.0 and align flutter version.

Enhancements and Corrections:

• Started using flutter_lints for checkstyle.