Remove mockito MethodChannel mocks from first-party plugins unit tests

[collinjackson]

Right now some of our first-party plugins tests use mockito to mock out MethodChannel and others use dependencies on mockito and some use setMockMethodCallHandler.

This results in contributors writing more tests in a deprecated test style (see flutter/plugins#1067) because they're adding to existing test suites or copying test code from other first-party plugins.

Here are the steps to remove dependencies on mockito:

• Update contribution guide with a note that the plugins team prefers tests written using setMockMethodCallHandler instead of mockito and may ask tests using mockito to be rewritten.
• Update firebase_analytics tests
• Update battery tests
• Update firebase_messaging tests
• Update share plugin tests
• Remove mockito pubspec dependency from firebase_admob
• Remove mockito pubspec dependency from firebase_database

[collinjackson]

This is no longer a thing since testing of platform interfaces is using mockito.

See https://github.com/FirebaseExtended/flutterfire/pull/1472/files

/cc @amirh

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