Add Open Measurement impression tracking

[ValentinPostindustria]

We have researched the Open Measurement SDK's behavior for impression tracking (burl) in the PUC.

OM SDK JS script contains the public methods that are available in all versions and are not obfuscated. We have researched different versions of AppLovin, GAM, and AdMob. List of these methods:

startSession
error
finishSession
publishAdEvent
publishImpressionEvent
publishVideoEvent
publishMediaEvent
publishLoadedEvent
setNativeViewHierarchy
setState
setDeviceVolume
init
setLastActivity

We can subscribe to all these methods using the proxy method. For receiving them we should use window.omidBridge.<method_name>.

We have tried to subscribe to publishImpressionEvent, but it works too fast and we can't handle it. So we decided to subscribe to the geometry change called by the setNativeViewHierarchy method. But this method can be called when a view is not on a screen or is not visible, so we need to check this method's parameter. Parameter options:

• gc_view_is_not_on_screen.json - checked by PR method viewIsNotOnScreen.
• gc_view_is_not_visible.json - checked by PR method viewIsNotVisible.
• gc_view_visible.json - case when we can track impression.

[bretg]

Thanks @ValentinPostindustria and @YuriyVelichkoPI -- while you're working on PUC tracking, please take a look at prebid/prebid-mobile-ios#1059