Fixed build warnings and imports. #689

Job	Run time
Build (java/advanced/APIDemo)	2m 18s
Build (java/advanced/BannerRecyclerViewExample)	2m 30s
Build (java/advanced/RewardedSSVExample)	11s
Build (kotlin/advanced/APIDemo)	4s
Build (java/admob/AppOpenExample)	2m 23s
Build (java/admob/BannerExample)	2m 15s
Build (java/admob/FullScreenNativeExample)	2m 50s
Build (java/admob/InterstitialExample)	2m 16s
Build (java/admob/NativeAdvancedExample)	2m 19s
Build (java/admob/RewardedInterstitialExample)	8s
Build (java/admob/RewardedVideoExample)	2m 36s
Build (kotlin/admob/AppOpenExample)	9s
Build (kotlin/admob/BannerExample)	6s
Build (kotlin/admob/InterstitialExample)	5s
Build (kotlin/admob/NativeAdvancedExample)	5s
Build (kotlin/admob/RewardedInterstitialExample)	5s
Build (kotlin/admob/RewardedVideoExample)	5s
Build (java/admanager/AppOpenExample)	2m 24s
Build (java/admanager/BannerExample)	2m 25s
Build (java/admanager/InterstitialExample)	2m 20s
Build (java/admanager/NativeAdsExample)	2m 26s
Build (java/admanager/RewardedInterstitialExample)	2m 28s
Build (java/admanager/RewardedVideoExample)	2m 24s
Build (kotlin/admanager/AppOpenExample)	8s
Build (kotlin/admanager/BannerExample)	4s
Build (kotlin/admanager/InterstitialExample)	4s
Build (kotlin/admanager/NativeAdsExample)	5s
Build (kotlin/admanager/RewardedInterstitialExample)	10s
Build (kotlin/admanager/RewardedVideoExample)	8s
	35m 31s

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