Move junitPlatform configuration into android namespace

[mannodermaus]

We have been using the junitPlatform configuration closure in the same place as the Java-based plugin, however it would be preferable to use either android.defaultConfig or android.testOptions in Android environments.

[mannodermaus]

Potentially, the resulting DSL could look like this:

android {
  testOptions {
    junitPlatform {
      jupiterVersion "..."
      vintageVersion "..."

      // Other properties from the Java-based plugin's extension
      // ...

      // Our existing parameters
      applyDefaultTestOptions true

      // New: Related to instrumented tests
      instrumentedTests {
        enabled true
        version "" // Override suggested default
      }
    }
  }
}

For starters, the top-level junitPlatform would redirect everything to android.testOptions, but log a deprecation warning as well.