Add Function to Initialize AdMob in Background

[InfoDevkota]

The initialize_on_background_thread() function is added to initialize AdMob on a background thread. I experienced a few ANRs on some low-end devices, and using background initialization along with the OPTIMIZE_AD_LOADING flag significantly improved responsiveness on my test device.

• Added initialize_on_background_thread() to handle AdMob initialization in the background.
• Emitted the admob_initialized signal upon successful initialization.
• Updated the readme to include optimization flags OPTIMIZE_INITIALIZATION and OPTIMIZE_AD_LOADING.

Official docs:

• https://developers.google.com/admob/android/optimize-initialization
• https://developers.google.com/admob/android/quick-start#initialize_the_mobile_ads_sdk

[Shin-NiL]

Thank you, my friend :)