Fix generated activity class name (namespace)

[friederbluemle]

Fixes incorrect activity component name generation when using applicationIdSuffix in build types. Previously, the plugin incorrectly prefixed activities with the suffixed applicationId instead of the actual package namespace where the code lives.

Problem

When an app uses applicationIdSuffix (e.g., .debug), the plugin was generating incorrect component names:

• App ID: com.example.debug (with suffix)
• Activity class: com.example.MainActivity (actual location)
• Before: -n com.example.debug/com.example.debug.MainActivity ❌
• After: -n com.example.debug/com.example.MainActivity ✓

Changes

• Use getNamespace() instead of getApplicationId() for constructing activity class names when activity starts with "."
• The -n flag now correctly uses: applicationId (with suffix) + activityClassName (using namespace)
• Added comprehensive unit and functional tests validating:
  • Activity name construction logic with namespace vs applicationId
  • Extension configuration and defaults
  • Both relative (.MainActivity) and fully qualified activity names