[MOD-2497] AdMob: Parity between Android and iOS

The iOS Swift rewrite in MOD-2495 already addresses a few parity issues between Android and iOS. This ticket is for changes required in the Android version of the module. Parity issues that need to be addressed: For the module proxy: * Add AD_SIZE_* constants * New method initialize to initialize the Google Ads SDK * Add debugIdentifiers property (see https://developers.google.com/admob/android/test-ads#enable_test_devices). This should replace requestTestAd from the banner view proxy. * Deprecate createView and replace with createBannerView * Remove Facebook audience framework. Not sure if this is possible on Android. We currently ship this by default for Facebook mediation. However i would like to remove it and users have to manually add it the their Project if they want to use it.. Should be easily done by just dropping it into a project's platform folder. There is no code setup necessary. * I would like to replace set/get tagForUnderAgeOfConsent with property isTaggedForUnderAgeOfConsent. In Swift this can be done with computed properties. Is this possible on Android? Banner view proxy: * Deprecate loadAd and requestAd and replace with load * Deprecate requestTestAd * Deprecate additional properties like prop_color_bg etc. Ad request customization should now be done via load * Add support for adSize option to specify the banner size. This is currently hardcoded to AdSize.Banner. * Remove deprecated properties testing and publisherId (Not sure for how long those are deprecated so maybe this needs to be done later) Interstitial Ad: * Add options for request customization to load * On iOS the proxy can be reused and a new interstitial ad is created each time "load" is called. Can we do the same on Android? General: * Deprecate the event name constants and just use strings. I don't think we can actually deprecate them in the code so we just need to add a note in the docs. New event names for banner and insterstials would be: ** ad_received => load ** ad_not_received => fail ** ad_closed => close ** ad_opened => open ** ad_left_application => leftapp