Workaround for apps crashing when FlutterActivity/Fragment

[mehmetf]

Description

When FlutterActivity/Fragment access bundle inferred parameters without initializing FlutterLoader, they crash. Let's fix that temporarily until #64458 is fixed.

Related Issues

flutter/flutter#64458

Tests

FlutterLoader does not have any tests.

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[xster]

Let me do it in #20769 (writing test).

Letting getAppBundlePath be not package visibility was a mistake I think since the only consumer is the FlutterActivityAndFragmentDelegate which always consumes it at the right time.

Defer everything until later.

[mehmetf]

Sounds good.