Previously, it was common for extensions to use something like package="::APP_PACKAGE::" inside the AndroidManifest.xml file. This would give the extension the same package name as the game itself. This is no longer allowed by the android build system, so each extension's AndroidManifest.xml file should have a unique identifier there, similar to how the game has com.yourcompany.gamename (until you change it to something else).
Stencyl's built-in extensions, as an example.
AdMob: package="com.byrobin.admobex"
Google Play Games: package="com.stencyl.extension.google"
Native: package="com.androidnative"
Purchases: package="com.stencyl.extension.purchases"
Note: Stencyl's built-in extensions can be found at [Stencyl install]/plaf/haxe/extensions for you to inspect them. And of course, your own extensions are at [stencylworks]/engine-extensions. If the extension has native Android capabilities, the AndroidManifest.xml file is found at [extension]/dependencies/[some identifier]/AndroidManifest.xml.