AdMob SDK release notes has some new requirement for 17.0.0 version:
Google AdMob publishers are required to specify a <meta-data> tag with key com.google.android.gms.ads.APPLICATION_ID in their AndroidManifest.xml. Refer to the Get Started guide for more details.
As I can see, everything work without adding additional <meta-data> tag in manifest, but I can also read on Get Started guide this text:
Important: This step is required as of Google Mobile Ads SDK version 17.0.0. Failure to add this <meta-data> tag results in a crash with the message: "The Google Mobile Ads SDK was initialized incorrectly."