This will be a short technical one for anyone else facing the same problem. I can't pretend to have a clue what I was doing here, only the procedure I followed in the hope it will help others, I found little information online on this subject.
I am writing an Android game and want to put in gamepad support, for analogue controllers. This had proved incredibly difficult, because the Android Studio emulator has no built in support for trying out gamepad functionality. So I had bought a Tronsmart Mars G02 wireless gamepad (comes with a usb wireless dongle). It also supports bluetooth.
The problem I faced was that the gamepad worked fine on my Android tv box device, but wasn't working under Linux Mint, let alone in the emulator, and wasn't working via bluetooth on my tablet and phone. I needed it working in the emulator ideally to be able to debug (as the Android tv box was too far).
Here is how I solved it, for anyone else facing the same problem. Firstly the problem of getting the gamepad working and seen under linux, and then the separate problem of getting it seen under the Android emulator (this may work under Windows too).
Under Linux
Unfortunately I couldn't get the bluetooth working as I didn't have up to date bluetooth, and none of my devices were seeing the gamepad. I plugged in the usb wireless dongle but no joy.
It turns out the way to find out what is going on with usb devices is to use the command:
lsusb
This gives a list of devices attached, along with a vendor id and device id (takes the form 20bc:5500).
It was identifying my dongle as an Xbox 360 controller. Yay! That was something at least, so I installed an xbox 360 gamepad driver by using:
sudo apt-get install xboxdrv
sudo xboxdrv --detach-kernel-driver
It still didn't seem to do anything, but I needed to test whether it worked so I installed a joystick test app, 'jstest-gtk' using apt-get.
The xbox gamepad showed up but didn't respond.
Then I realised I had read in the gamepad manual I might have to switch the controller mode for PC from D-input mode to X-input. I did this and it appeared as a PS3 controller (with a different USB id), and it was working in the jstest app!! 
Under Android Emulator
Next stage was to get it working in the Emulator. I gather the emulator used with Android Studio is qemu and I found this article:
https://stackoverflow.com/questions/7875061/connect-usb-device-to-android-emulator
I followed the instructions here, basically:
Navigate to emulator directory in the android sdk.
Then to run it from command line:
./emulator -avd YOUR_VM -qemu -usb -usbdevice host:1234:abcd
where the host is your usb vendor and id from lsusb command.
This doesn't work straight off, you need to give it a udev rule to be able to talk to the usb port. I think this gives it permission, I'm not sure.
http://reactivated.net/writing_udev_rules.html
Navigate to etc/udev/rules.d folder
You will need to create a file in there with your rules. You will need root privileges for this (choose to open the folder as root in Nemo or use the appropriate method for your OS).
I created a file called '10-local.rules' following the article.
In this I inserted the udev rule suggested in the stackoverflow article:
SUBSYSTEM!="usb", GOTO="end_skip_usb"
ATTRS{idVendor}=="2563", ATTRS{idProduct}=="0575", TAG+="uaccess"
LABEL="end_skip_usb"
SUBSYSTEM!="usb", GOTO="end_skip_usb"
ATTRS{idVendor}=="20bc", ATTRS{idProduct}=="5500", TAG+="uaccess"
LABEL="end_skip_usb"Note that I actually put in two sets of rules because the usb vendor ID seemed to change once I had the emulator running, it originally gave me an UNKNOWN USB DEVICE error or some such in the emulator, so watch that the usb ID has not changed. I suspect only the latter one was needed in the end.
To get the udev rules 'refreshed', I unplugged and replugged the usb dongle. This may be necessary.
Once all this was done, and the emulator was 'cold booted' (you may need to wipe the data first for it to work) the emulator started, connected to the usb gamepad, and it worked!
This whole procedure was a bit daunting for me as a linux newbie, but if at first you don't succeed keep trying and googling. Because the usb device is simply passed to the emulator, the first step getting it recognised by linux itself may not be necessary, I'm not sure. And a modified version of the technique may work for getting a gamepad working under windows.
