MoPubNative should not depend on Activity

mopub
android

#1

MoPubNative:68 (https://github.com/mopub/mopub-android-sdk/blob/master/mopub-sdk/src/main/java/com/mopub/nativeads/MoPubNative.java#L68) requires an Activity but what it really needs is a Context. The current implementation prevents developers from sharing a MoPubNative instance across different Activities.