How to generate debug.keystore for the native android library ?

This is the newest update:

With Android Studio 2.3 (maybe earlier version do the same thing), ~/.android/debug.keystore will be created for you at your first debug build and the alias/pass for debug key is androiddebugkey/android.

If you want to replace the Flash Builder default keystore, you should customize the launch by replace the parameters:

-keystore HOME/.android/debug.keystore -storetype jks -storepass android -alias androiddebugkey

Don't forget to clean your workspace after re-configure.

Maybe is too late but i'll find the way on Adobe flash builder 4.7:

Debug->Debug-Configuration->Customize launch

here you can see the parameter [-keystore] and get the debug-certify  (in my case: C:\Program Files\Adobe\Adobe Flash Builder 4.7 (64 Bit)\eclipse\plugins\com.adobe.flexide.multiplatform.android_4.7.0.349722\resources\debug-certificate-android.p12  )used for flash builder debugs release!

notice that certify CAN be changed .  In case you need the store pass the default is "debug"  or you can get it on the "show Command" button 

By the way ( it seem you are using Google API ) make sure you are using the proper package name: when you release an [-target apk-debug] from ADT command line your APK will be modified your <id> package from <id>com.example.application</id> to  <id>com.example.application.debug</id> so, make sure to include the ".debug" prefix on the Google's console if you need it.

Happy codding