VKSdk class improved, double checked locking singleton for thread safety calls to VKSdk.initialize(), cleaned code style
VKSdk class improved, double checked locking singleton for thread safety calls to VKSdk.initialize(), cleaned code style

jintoga/vk-android-sdk,rcsir/vk-android-sdk,bossvn/vk-android-sdk,Vittt2008/vk-android-sdk,VKCOM/vk-android-sdk,vknet/vk-android-sdk,Heeby/vk-android-sdk,VKCOM/vk-android-sdk