How can I find out which library defines an Android class?

The Context

I need to do some reverse engineering on a handset [running Android 6.0] for which the system source code is not available. All I know about the code in question is the Java package name, e.g. "com.company.custhardware", the names of several classes of interest under that package e.g. "com.company.custhardware.HwOperations", and the fact that these classes of interest are imported and used by system apps.

The classes of interest are also available for discovery/use by normal apps via reflection.

The Question

How can I locate the file(s) where these classes are defined?

My best guess would be /system/framework/framework.jar, but that jar appears to be empty (except for a META-INF/MANIFEST.MF file) on this device.