One of the reasons Apple has been requested to build a backdoor into iOS is because the FBI can't access the data on a certain iPhone directly. To get into the encrypted device, they need the correct PIN. Then they'll be able to access the data they want.
By default, all devices running iOS 8 and above are encrypted. Whether it's your new iPhone 6s or your grandmother's iPad 2, if it has the OS installed, the content on it has been encrypted to some degree. The best way to see how many iOS devices are encrypted is thus by seeing how many are using those operating systems.
Directly from Apple, we can see the breakdown of iOS usage across all iOS devices. All iOS devices running iOS 8 and up are encrypted, thus leading to the conclusion that 94% of all iOS devices have encryption enabled. I would say 'by default', but Apple's control of the operating system and limitations to the user prevent someone from 'disabling' that encryption.
Android, meanwhile, can't boast those same numbers. In fact, it's a lot harder to make a proper estimate of how many Android devices are encrypted. CNN's Jose Pagliery:
Google introduced encryption on Android in 2011, but it was buried deep within a phone's settings. Not until late 2014 did Google begin asking customers if they wanted to encrypt their phones during the setup process.
Although 97% of Android phones have encryption as an option, less than 35% of them actually got prompted to turn it on when they first activated the phone. Even then, not everybody chooses that extra layer of security.
While the option has been there, most users simply don't enable device encryption. It was enabled by default with the Lollipop update. And while Android's next version, Marshmallow, requires encryption to be enabled, the number of devices currently running it is only 2.3%. And even then, such encryption is only enforced on "high-performing devices". With the plethora of Android devices available out there, it's likely that consumers are not always buying or owning such a device.
If the San Bernardino terrorists were using Android devices, we probably wouldn't have seen any government requests at all. Chances are the device would either be on an older OS version or simply not be encrypted at all. If this had been the case, this situation could've been a lot different than it has played out.