Android support

[lynn]

Adds Android support to the gamepad plugin.

@apecoraro @nir-ziv To test:

• Build the example app in this package on your Android device.
• Connect a Bluetooth gamepad to your device.
• Right before the first registered button press, you should see a "connected gamepad" event in the example app.
• Test all the buttons: A, B, X, Y, shoulders, stick buttons, Menu, and the directional pad. They should all produce log entries in the app. The B button should not close the app.
  • ("Select/Back" will always perform a UI action on Android, I'm afraid.)
• Test the joysticks and trigger buttons. Moving them should also produce log entries in the app.
• Test the "Call gamepads()" button in the example app: your controller's name should show up.

Basically, you should see something like this:

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