Fix banner load callbacks firing before creative attachment

[mdanylov-sigma]

#1220

Summary

Fixed the banner/custom renderer load callback ordering on both iOS and Android so the load callback is not fired before the creative view is attached.

Android

Updated PrebidDisplayView.java so adLoaded marks the load notification as pending instead of notifying immediately.

When viewReadyForImmediateDisplay is called, the creative is attached first, then pending onAdLoaded is fired, followed by onAdDisplayed.

Added regression coverage in PrebidDisplayViewTest.java to verify that:

• onAdLoaded is not fired before the creative is ready
• the creative is already attached when onAdLoaded fires
• onAdLoaded still fires before onAdDisplayed