Add didInjectView callback to PrebidMobilePluginRenderer

[MattMur]

Summary

This PR addresses a few issues I ran into while working with the plugin renderer system.

1. getPreferredPluginRendererName() returns null by default
When a bid response doesn't include a renderer key in its meta, getPreferredPluginRendererName() is returning null. This leads to a live production bug where the if condition in BannerView.displayPrebidView that can never be hit, since by default it will always evaluate to false.

Fix: Set PrebidRenderer as the default for getPreferredPluginRendererName()

2. Final layout responsibility should be given to chosen PluginRenderer
Layout parameters for the injected ad view are being set directly inside BannerView. Since BannerView doesn't have visibility into how a given renderer intends to display its content, this feels like the wrong place for this logic. The renderer itself is best positioned to own those decisions — it knows the ad format, creative dimensions, and any renderer-specific requirements.

Fix: Add new callback to the PrebidMobilePluginRenderer to notify it once the ad has been injected so it can manage the final layout.

3. MATCH_PARENT is the wrong default for fixed-size banner ads
Due to the bug from issue#1, MATCH_PARENT is always being set on the injected view. This is a potential source of issues since ideally the PrebidRenderer wants the size to be controlled by logic in PrebidWebView, and the outer DisplayView should simply WRAP_CONTENT.

Fix: Use the new didInjectView method on PrebidRenderer to set WRAP_CONTENT with center gravity.

Testing

I've tested these fixes on the PrebidInternalTestApp using every "Banner" "In-app" variant, and each one looks and behaves as expected.

Parallel PR for iOS PR 1261

[postindustria-code]

Please, add unit tests covering updated behaviour. Everything else LGTM

[YuriyVelichkoPI]

@MattMur will you be able to add tests? If you don't have time for it now, let me know and we create follow-up ticket and merge this one