According to official Android documentation, you should always put .jar files only in the /libs folder.
See official documentation
raw /
For arbitrary raw files. Saving asset files here is essentially the same as storing them in the assets / directory. The only difference is how you refer to them. These files are processed by the aapt command and there must be a link from the application using the resource identifier in the R class. For example, this is a good place for media such as MP3 or Ogg files.
libs /
Contains private libraries. The module is stored in the main application.
Once you have placed the .jar file inside the /libs folder, you can easily reference it through the gradle.build file,
For example, I put this acra-4.6.2.jar file in the /libs folder

and made a link in the gradle.build file as follows:
dependencies { compile fileTree(include: ['*.jar'], dir: 'libs') compile files('libs/acra-4.6.2.jar') }
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