Fixed build warnings and imports. #709

Job	Run time
Build (java/advanced/APIDemo)	2m 28s
Build (java/advanced/BannerRecyclerViewExample)	2m 21s
Build (java/advanced/RewardedSSVExample)	5s
Build (kotlin/advanced/APIDemo)	9s
Build (java/admob/AppOpenExample)	2m 24s
Build (java/admob/BannerExample)	2m 31s
Build (java/admob/FullScreenNativeExample)	2m 13s
Build (java/admob/InterstitialExample)	2m 18s
Build (java/admob/NativeAdvancedExample)	2m 19s
Build (java/admob/RewardedInterstitialExample)	4s
Build (java/admob/RewardedVideoExample)	2m 25s
Build (kotlin/admob/AppOpenExample)	5s
Build (kotlin/admob/BannerExample)	5s
Build (kotlin/admob/InterstitialExample)	5s
Build (kotlin/admob/NativeAdvancedExample)	4s
Build (kotlin/admob/RewardedInterstitialExample)	11s
Build (kotlin/admob/RewardedVideoExample)	11s
Build (java/admanager/AppOpenExample)	3m 6s
Build (java/admanager/BannerExample)	2m 19s
Build (java/admanager/InterstitialExample)	2m 27s
Build (java/admanager/NativeAdsExample)	2m 26s
Build (java/admanager/RewardedInterstitialExample)	2m 25s
Build (java/admanager/RewardedVideoExample)	3m 28s
Build (kotlin/admanager/AppOpenExample)	10s
Build (kotlin/admanager/BannerExample)	4s
Build (kotlin/admanager/InterstitialExample)	5s
Build (kotlin/admanager/NativeAdsExample)	5s
Build (kotlin/admanager/RewardedInterstitialExample)	5s
Build (kotlin/admanager/RewardedVideoExample)	4s
	36m 42s

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