[shared_preferences] Remove duplicate integration test (#9368)

We accidentally enteded up with two identical, but differently named, Android integration test harness files in `shared_preferences`, causing CI to run the integration tests twice. This removes the MainActivityTest.java version, leaving the better-named FlutterActivityTest.java (there is no MainActivity in the app, it uses FlutterActivity directly).

Fixes flutter/flutter#162266

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: stuartmorgan-g